Hi @Mcd11, sorry to hear about that. 
Do you know what they did at the service centre to repair the device? Usually you get a paper from the service centres where you can read about what has been done.
Is the device detecting the SIM card at all and have you tried with a different SIM card in the device? If the SIM card is detected, go to Settings > Network & Internet > Mobile network > Network operators and see if the search finds any networks > register on the one with a house icon.
Let me know how it goes!
