@Router.1 Is the sim card detected by the M7350? If it was deteced but no IP address, you can try to create the profile manually. Or you can follow the instruction below to do some troubleshooting....

@grt You can try to degrade the firmware to have a try. The firmware which is older than the current one is optional. https://www.tp-link.com/support/download/tl-mr6400/#Firmware If still the same,...

@JoSt please refer to https://community.tp-link.com/en/home/forum/topic/164924?page=1&t=2019%2F07%2F12%2007%3A13%3A00%20%2B0000

@JoSt Please ensure that the sim card support data roaming and enable this feature on the web UI of M7450. How to login to the web interface, click here Besides, you can create the profile manually...

@credo What is the hardware and firmware version of the VR600? Please show us a screenshot of the error message when you tried to disable traffic monitor.

@YaKoO The TD-W8970 supports 3/4G router mode, while there may have some compatibility issue between the 3/4G dongle and router. You can refer to the link below....