Lg v20 with no cel signal after unlock

[AC Question]

Hey, I need some urgent help with my phone. I bought a great condition lg v20 of eBay, it was sold as unlocked but when it arrived and switched on was asking for an unlock code. I took it to a supposed cellphone expert who tried to unlock it with some kind of small box, I suppose with unlocking software but not with an unlock code. After finishing the unlocking process I have no cel signal with my 4g lte sim, just continually searches for cel signal. We tried to get a signal with 5 different SIM cards with no luck, the strange thing is with an old 3G sim it picked up a signal and managed to make a call. I took to another guy who said the unlock they tried could have damaged an important android security file and there is no solution. I have no idea what to do now, feeling really desperate. Any suggestions or comments would be really appreciated!

 

[B. Diddy]

Welcome to Android Central! What's the exact model number for the phone, and which carrier are you on? Are you certain the phone supports the carrier's network bands?