Why mi logic board Does not grab signal

I have a s7 edge model sm935FD (dual sim from europe) apparently it is damaged the logic board since it restarts constantly, I got an s7 edge sm935P (sprint e.u.a) and when exchanging logic boarda the sm935fd grabs signal but the sm935P does not grab, in summary The sm935FD has signal on both devices but the sm935P only in its device grasps signal, I would like to know what part I should take of the sm935P in addition to the logic board to make it work