Unhappy [Q] how to fix "not registering network on i747"

Hey I have i747 Galaxy unlock. I bought a sim and it was working until i got home. When i got home the signal totally dropped ..When i got the school it picks up back again at certain parts of school. How can i fix this. I am not a techy . so i would need a step by step guide. I have already rooted the device. I am in Jamaica an di bought the ohone in the States and unable to send it back. What should i do.
I already tried to pulling the battery and removing the sim.
I tried changing to GSM only
I tired restoring the phone and nothing works
my friend was also trying to flash a modem on but it deletes the efs files. he said he tried to back up the efs file and it still not working.
i am in a corner here