How to configure a mini pcie huawei me909s-120p V2 module in OpenWrt

Your modem is in ECM mode, so you don't need to switch it and the required drivers are loaded.
The comm port you should use for commands is /dev/ttyUSB2

Please refer to an older thread about the same modem: We826 + Huawei me909 (s-120) - trying and failing to get up and running