Preparing the Modem: Standalone Ethernet (PHY) Mode

Before you install QManager, the modem has to run in standalone Ethernet mode. This guide explains why that matters, lists the AT commands for each modem family, and shows how to confirm the connection and switch back if needed.

Why Standalone Mode#

By default, a Quectel modem runs in a TE ("terminal equipment") mode. In this mode the modem is a peripheral attached to a host over USB (NDIS/QMI or MBIM) or PCIe, and the host is in charge. On a router, the router's drivers and ModemManager or QMI scripts dial the data call, keep it up, and re-dial it whenever it drops.

That host-driven arrangement is the fragile part. Router-side scripts and drivers vary by vendor and firmware, and a router reboot or a driver hiccup can take the data call down. Some users have found this setup unreliable.

Watch what happens to the data call in each setup.

Router-driven

Default · TE mode

usbnet 0 or 2 · pcie/mode 0

Data call status: Data call up

Cell tower
5G / LTE
Modem
Waits for the host to dial it
Connected through Radio.
Router
Needs modem drivers plus QMI/MBIM scripts
Connection manager: Owns the data call
Connected through USB or PCIe endpoint · QMI / MBIM control.

Router reboots or a driver hiccups: the data call drops until the router dials again.

Standalone Ethernet

Required for QManager · AP mode

usbnet 1 · pcie/mode 1

Data call status: Data call up

Cell tower
5G / LTE
Modem
Runs its own Linux, DHCP server and QManager
Connection manager: Owns the data call
Connected through Radio.
Ethernet PHY
RTL8125 on the M.2-to-Ethernet board
Connected through PCIe Root Complex.
Router WAN port
Just asks for an IP, like any ISP uplink
Connected through Ethernet cable · DHCP.

Router reboots: the modem stays online and the WAN link is back as soon as the port comes up.

The difference is where the data call lives. Standalone mode moves it out of the router and onto the modem.

In standalone mode (AP initiator), the Linux system running on the modem itself dials and maintains the cellular connection. The modem runs its own DHCP server and exposes a plain Ethernet port through the board's PHY, such as an RTL8125. The router connects to that port like any other ISP uplink and requests an address. It needs no modem drivers, and the modem is already connected when the router starts up.

Cameron Thompson's Cellular Modem Wiki makes the same point: "the modem's onboard connection management often works better than the scripts many routers use."

Removing the router from connection management removes a whole variable, which is why standalone mode is more stable. It is also the only arrangement QManager supports, because QManager runs on the modem itself.

Aspect	Router-driven (TE)	Standalone (AP)
Who dials the data call	The router, through its modem drivers and scripts	The modem, which keeps the connection up on its own
What the router needs	QMI or MBIM drivers plus dial and keep-alive scripts	A DHCP client on its WAN port. No modem drivers or scripts
What happens when the router reboots	The data call drops and has to be re-dialed	The modem stays connected
Where QManager runs	Not supported	On the modem itself
Modem modes	`usbnet` 0 or 2, `pcie/mode` 0	`usbnet` 1, `pcie/mode` 1

Where This Fits in Setup#

Standalone mode is step 2 of the setup:

Unlock ADB access so you can reach the modem's shell later. The AT commands below only need a USB serial connection.
Switch the modem to Ethernet mode (this page).
Set your APN and verify the data call.
Mount the modem in the Ethernet board and plug the board into the router's WAN port.
Install QManager with the RM551E guide or the X5*/X6* guide.

Send the AT commands with the modem connected to your computer over USB. You can use Quectel's QNavigator with "automatic initialization" unchecked, or any serial AT terminal.

X5/X6 Modems (RM520N, RM521F, RG501Q, RM50xQ)#

These commands apply to Snapdragon X5*/X6* modems, which use the SDXLEMUR (X62/X65) and SDXPRAIRIE (X55) platforms. Send them in order:

AT+QCFG="data_interface",0,0
AT+QETH="eth_driver","r8125",1
AT+QCFG="pcie/mode",1
AT+QCFG="usbnet",1
AT+CFUN=1,1

Command	What it does
`AT+QCFG="data_interface",0,0`	Keeps the host data and diagnostic interfaces on USB (`0` = USB, `1` = PCIe), so the PCIe bus is free to drive the Ethernet chip.
`AT+QETH="eth_driver","r8125",1`	Enables the driver for the RTL8125 2.5GbE chip on the board. X5/X6 modems need the endpoint driver enabled after switching to RC mode. To see the current driver and the supported list, send `AT+QETH="eth_driver"`.
`AT+QCFG="pcie/mode",1`	Sets PCIe Root Complex (RC) mode. The modem becomes the PCIe host and drives the Ethernet chip as its endpoint. The default, `0`, is endpoint mode, where a host drives the modem.
`AT+QCFG="usbnet",1`	Sets USB ECM, an AP mode in which the modem manages its own connection.
`AT+CFUN=1,1`	Reboots the modem to apply the changes.

The modem reboots after the last command. Wait for it to come back before you continue.

RM551E (Snapdragon X75 / SDXPINN)#

On the RM551E, send these three commands in order:

AT+QCFG="pcie/mode",1
AT+QCFG="usbnet",1
AT+CFUN=1,1

This sequence is shorter than the X5*/X6* one. Cameron's SDXPINN guide uses only these three commands. There is no data_interface line and no AT+QETH="eth_driver" line, Cameron's M.2 form factor notes list only the RM50x, RM52x and RM530 as needing the endpoint driver enabled after switching to RC mode.

Set the APN and Verify the Connection#

Set the APN for PDP context 1, then reboot the modem:

AT+CGDCONT=1,"IPV4V6","your-apn"
AT+CFUN=1,1

Replace your-apn with your carrier's APN. After the modem comes back up, check the connection:

Command	What to look for
`AT+QSIMSTAT?`	SIM detected. `0,1` means yes.
`AT+QENG="servingcell"`	`NOCONN` is normal. `LIMSERV` usually means the APN is wrong.
`AT+QMAP="WWAN"`	Connected when you see `+QMAP: "WWAN",1,1,"IPV4","<address>"`. The first `1` means connected with an IP address. The second `1` is the PDP context in use.
`AT+CGDCONT?`	The APN set for each PDP context.
`AT+QCFG="pcie/mode"`	Reports `1`.
`AT+QCFG="usbnet"`	Reports `1`.

A default MBN profile can override your APN after a reboot, so check AT+CGDCONT? again after rebooting. The data call uses PDP context 1 by default.

Optional: IP Passthrough#

By default, the modem acts as a NAT router for IPv4. On X5*/X6* modems its LAN gateway is 192.168.225.1. Under QManager on the RM551E it is 192.168.224.1. Your router therefore receives a private address behind the modem, which is double NAT. IP passthrough hands the cellular address straight to the router and removes the second layer of NAT.

To enable passthrough on an X5*/X6* modem:

AT+QMAP="MPDN_rule",0,1,0,1,1,"FF:FF:FF:FF:FF:FF"

The fields are:

Rule: 0
PDP context: 1
VLAN: 0 (none)
IP passthrough mode: 1 (Ethernet)
Auto-connect: 1 (on)
MAC address: "FF:FF:FF:FF:FF:FF" passes the IP to the last connected Ethernet device.

The modem's gateway stays reachable while passthrough is on.

To turn passthrough off:

AT+QMAP="MPDN_rule",0
AT+QMAPWAC=1

Releasing the rule briefly shuts off the Ethernet port, which then comes back up. AT+QMAPWAC=1 is only needed if the modem is not in ECM mode (usbnet 1). You can optionally reboot with AT+CFUN=1,1 afterwards.

Reverting to Router-Driven Mode#

To return the modem to its default router-driven mode, send:

AT+QCFG="pcie/mode",0
AT+QCFG="usbnet",0
AT+CFUN=1,1

QManager will not work in this mode.

Credits & Sources#

The commands and behaviour in this guide come from Cameron Thompson's Cellular Modem Wiki. The specific pages used are:

SDXLEMUR: M.2 to Ethernet
SDXPRAIRIE: M.2 to Ethernet
SDXPINN: M.2 to Ethernet
M.2 Form Factor Modes
SDXLEMUR Advanced Configuration
Verizon Connection Notes, the source of the RM551E rule caution