I can’t reproduce this, but here is how I would troubleshoot it and why:
Put the device in AirPlane mode and then power off.
Start up and then go to settings (still in AirPlane mode) and look at the cellular data information / carrier information / phone information. Perhaps record/screen shot the MEID/IMEI and SIM information.
Open and re-seat your SIM ...
Try it & see.
Probably nothing will happen unless they have a partnership agreement with your own network - which usually applies more to roaming, abroad & would automatically switch if you just left your Network set to Automatic.
If you have no contract with the other network, they will not feel obliged to connect you.