First thing to try is just to load the correct modem to the phone for the firmware it currently has. If it has a custom rom, then would be best just to just to restock the thing with Odin. You'll find tons of info here:
There's a section called Rom Parts that has a collection of modems to use, depending on the region the phone is originally from. The wiki has all the links to the Odin version you need, drivers, guides, etc.
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