I don't know the answer to that, since according to Google Sasktel is CDMA based and we don't have those here. I thought CDMA handsets only used a SIM for 4G, so if it doesn't connect to anything then my guess is that it's not registered with any of the networks. But as I say, only a guess because that technology isn't used in Europe so I have no experience.
Maybe you need to phone HTC if they've not replied to email. If it's CDMA I'd think the network will need the MEID to register it (but they should have asked that, surely?). Or maybe Facebook guy can tell you the phone's history, if it was originally on a different network (the most obvious reason it would change when HTC repaired/replaced it)?