Ok, here go a few results of my experimentation with two different 4G dongles, a modem and a router, plugged into the USB port on the back of the VL2.
1. Modem D-Link DWM-222 4G LTE. It is not detected by VL2 at all, but it becomes impossible to select any of the OEM screens while plugged in, and the VL2 will only display its own home screen, without any other possibility, even to connect Android Auto.
2. Router Huawei E8372h-320. It is detected, and provides internet access, but generates the same issue with the selection of the OEM screens described above. Also, it creates an issue that makes it impossible to start Android Auto in association with a VPN, although the AA app is whitelisted o nthe phone. This latter problem disappears disconnecting the router. Not sure why as I could not make Wireshark work on the VL2 with the router connected.
Are any of the GROM technical experts familiar with this, or aware of it ?