Default [Q] need help with s4 sprint

hi guys have a nice day

i have a sprint s4 4.2 and after unlocking it for international use the device by downgrading the modem from MF9 to MDC
and after complete unlocking i put my gsm simcard on it ( not US simcard)
the signal is very very week inside my house and mostly without any signal . or it just return full signal after while but then directly get down until disappear .

today i flash back the MF9 modem back to see if is there any different.
but actually it is the same thing
and i have 2 other phone samsung galaxy s2 and galaxy ace all of them has good signal except my s4 sprint

please help me with this problem