I was able to find the answer to this after some further search.
1. Get serving system info, to check the connected network
root@turris:~# uqmi -d /dev/cdc-wdm0 --get-serving-system
{
"registration": "registered",
"plmn_mcc": 260,
"plmn_mnc": 2,
"plmn_description": "TM PL",
"roaming": true
2. Initiate network scan, to check the available networks and find the mcc and mnc number, needed for point 3:
root@turris:~# uqmi -d /dev/cdc-wdm0 --network-scan
{
"network_info": [
{
"mcc": 260,
"mnc": 2,
"description": "TM PL",
"status": [
"current_serving",
"roaming",
"not_forbidden",
"preferred"
]
},
{
"mcc": 262,
"mnc": 2,
"description": "Vodafone.de",
"status": [
"available",
"roaming",
"not_forbidden",
"preferred"
]
}
3. Register at specified network, get the mcc (Mobile Country Code) and mnc (Mobile Network Code) from the desired network in order to lock the modem into.
root@turris:~# uqmi -d /dev/cdc-wdm0 --set-plmn --mcc 262 --mnc 2
4. Check if the changes were successful:
root@turris:~# uqmi -d /dev/cdc-wdm0 --get-serving-system
{
"registration": "registered",
"plmn_mcc": 262,
"plmn_mnc": 2,
"plmn_description": "Vodafone.de",
"roaming": true