Do you mean a *replacement* SIM rather than a *new* SIM?
A new SIM will, by default, expect to be activated on a new account although you can probably contact 48 support to activate it as a replacement on your existing account.
But apart from that, you really should contact 48 support about your network connectivity issue as they may be able to sort it out without any need for any new/replacement SIM at all.
See the link in my signature below for how to contact support.
Hope this helps.