Dad had a similar issue, he was on Mobilink.
He got a new SIM issued and everything was fine.
Phone has been blacklisted most probably.
Try any other SIM in it, if you still can't connect to a network then it's blocked.
Same thing happened with my S5, one day it just stopped connecting to Warid.
I tried Telenor as well but it wouldn't connect.
Turned out it was blacklisted
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