I would bet the IMEI was blocked somehow or the antenna is bad
Check the IMEI, if it is blocked, the only legal way to fix it is to swap the logicboard(I would suggest changing it, but it turns out it's not legal, even if it is done as a personal fix and you aren't selling your device, DO NOT DO THAT!)
By all means if it isn't, the 3G antenna could be bad, change that
If it still doesn't help, your only hope is a logicboard(legalities changing the IMEI)