First of all, you need a SIM card that allows connections to other network providers, such as the“ASCEND Router SIM card“.
If you have this, it is relatively simple.
Go to the web interface of your router at
Network –> WAN –> Cellular (or similar)

In the menu that now opens, you will find the“Carrier Selection” option relatively in the middle. Press the“question mark” here

In the following dialog please click on“here“

Then please select“Manual Select“

Please click on“Scan” in the following pop-up window

CAUTION: This module will go offline for the duration of the network search. If this is your only Internet connection and you perform this activity remotely (e.g. using InControl access), you will lose the Internet connection. It is therefore advisable to only do this if you have at least one other “healthy” Internet connection via which you can access the device or if you access the router locally via LAN.

Once the scanning process is complete, you can select the network provider and the card will now connect to it.