Add documentation for setting up QMI based cellular device on OpenWRT

Tested on TP-Link TL-WR902ACv3 (MT7628) with OpenWRT 25.12 and
Quectel EC25-EUX

doc/OpenWRT/OpenWRT-qmi-setup.md

@@ -0,0 +1,35 @@
+## Check for QMI modem
+Check if a device has been recognized as /dev/cdc-wdm0:
+```
+ls /dev/cdc*
+cdc-wdm0
+``` 
+
+Check if the device is detected:
+```
+cat /sys/kernel/debug/usb/devices
+```
+
+If the device is already in QMI mode you can proceed with
+the installation of the kernel module.
+
+## Install USB QMI Kernel module
+```
+apk update
+apk install kmod-usb-net-qmi-wwan uqmi luci-proto-qmi
+```
+
+Reboot the device, so everything gets loaded.
+
+## Setup the OpenWRT interface LuCI: 
+Go to "Interfaces"
+Click on "Add"
+Select "Protocol" -> "QMI Cellular"
+Add "Device" -> "wwan0" or "/dev/cdc-wmd0"
+
+### For Aldi Talk only
+APN = "internet.eplus.de"
+Authentication Type = "PAP"
+username = "eplus"
+password = "internet"
+And don't forget your SIM pin if one is set.