Is "SIM not registered" the full verbatim error message or is it something else?
Does the phone let you do a manual search for networks?
If so, can you find and connect to 48?
Is the phone unlocked for use on any network?
You'll probably have to contact 48 support about this.
See the link in my signature below or email them at email@example.com
You might also want to try ordering a new SIM just to see if that makes any difference.