How do I select the network provider of the SIM card for my Peplink router?

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 these, it is relatively simple.
On the web interface of your router, go to
Network --> WAN --> Cellular (or similar)

In the menu that now opens, you will find the option"Carrier Selection" 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 carry out this activity remotely (e.g. using InControl access), you will lose the Internet connection. It is therefore only advisable to 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.