Hi,
- Look at this manual:
http://www.formymobile.co.uk/desiredisassembly.php#.U8z0UPmSxdM
The GSM antenna is the small module at the pic 2. Check and clean carefully the contacts (2x).
- Check at the "Settings" - Network connection. If its - manual, change it to automatic.
- Change the network mode to GSM only.
- You can do a factory reset to resolve the problem, if it's software.
If this not help you, ask a specialist for help.
Regards.