The FRITZ!Box 6850 5G cannot be operated simultaneously via an external modem and the integrated SIM card in order to automatically switch to the other connection if the connection is interrupted.

The router has an integrated 5G modem and supports Internet access via mobile radio (SIM card). There is no indication that it can be used as a pure router behind an external modem with automatic fallback function to the SIM card. Use as a backup solution in the event of failure of the external modem is therefore not planned or documented.