My Xperia SP won't switch to GSM network

[lordMAC]

I bought an SP yesterday and since then the network only connects to 3G but keeps saying no network found on GSM. I have tried three different network carriers to no avail. Anyone knows a fix for this??

[Uliwooly]

Settings > more > mobile networks > network mode > GSM only 

Don't forget the Kudos 

[lordMAC]

tried that already....signal goes out completely

[Uliwooly]

Try a different SIM card