if you have unknown or Null IMEI installing a rom wont fix it.
1) click on this link and download the drivers first: http://forum.xda-developers.com/show....php?t=2038555
2) Download and install kies from this URL: http://www.samsung.com/in/support/us...tware/KIES/JSP
3) power down your device, once its off, hold down the volume button down - home button - power button, hold all three at the same time. Should ask you to confirm enter to download mode.
4( Launch kies and do not plugin your device until it says so
5) Once in kies go to the menu and bar and click on "tools" option and choose "Firmware upgrade and initialization
6) Enter your model number, IE SGH-T889
7) Enter serial number, can be found back of the box or under the battery
8),follow the on screen prompts to connect your device and restore it.
9) once its done installing and booted up you should have your signal back.
Good luck hope it works for you.