have n AT&T smsung galaxy s2 1777??? with4.0.4on straight talk and no service!!! please help

[HVAC Mech]

I recently bought an AT&T Samsung galaxy s2 1777 with the 4.0.4 on it, bought the B.Y.O.P. package at wal mart used the gray sim card to activate with a new number. and cant get little if any service in the same area as always that my HTC inspire with an old AT&T sim would get awesome reception, before it was stolen. so what do I need to do, use another card, reactivate this one, reboot the phone, change some of the setting, or what? please help.