HTC One M8 - Won't register on network - IMEI number valid and no issues

Hi,
I have an issue with my HTC One M8 (bought via gumtree)
I have entered a new sim card into the phone which was on 02 with its last owner; when I try to put my sim card in I can get internet mobile data but cannot call or text etc.
When manually trying to connect to a network the phone says your sim card does not allow a connection to this network but then sometimes it says connected and I can get on apps and internet but can't call or text.
I have tried factory reset, different sim, making sure APN is correct but still no luck!
I also checked the IMEI encase I was sold a stolen phone but the IMEI check came back with no issues.
Any ideas anyone?