Sounds like the action of removing the old sim may have damaged the sim carrier in the phone.
Firstly I would try a known working sim just to be sure, then if still not working, try your local market phone repair stall for a new sim reader.
I have no affiliation whatsoever with O2 or any subsidiary companies. Comments posted are entirely of my own opinion. This is not Customer Service so we are unable to help with account specific issues.
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