Check the IMEI to see if it's blocked. You can find many IMEI checkers online. If it is blocked, you're nothing short of screwed. Swappa has a IMEI/ESN check you can use.
DO NOT PATCH THE IMEI! It's probably illegal in your part of the world. As such, you need to look into this on your own. In the US, I know it's illegal unless you do it in a certain way. Even if you found a legal way to do it, you will probably run into more issues later on. Most carriers reject IMEI swapped phones. Samsung and many other phones validate the IMEI to protect against this practice. It's probably illegal in your part of the world anyway. If they see this, it fails the IMEI certification and will not run the radio. I do not recommend this practice one bit.
If the IMEI checked clean, then it's a antenna assembly issue. You'll need to replace it.