Phone unlocked, SIM not working? HTC One (M7)

[AC Question]

So, I got my unlock code to open my device to other networks and everything seemed to be fine. I was on EE and unlocked the HTC One M7 successfully. After putting my O2 sim card into the phone it comes up on the top right with no service (5 feint bars with an X above them) Does anybody know whether i need to do anything extra to get the phone working with the O2 sim? I'm so lost and any help would be appreciated. Thankyou