That sounds like WI-FI chip is damaged or dry soldered.There have two factor that you can solve by this way.
Sometime WI-FI chip dry soldered throw over heat that remove PCB from board and re-hot WI-FI chip on 240 aprox 1 min then check most of the cases are solved.If the problem is still same remove the WI-FI chip then re-ball and fix.
If you have to change the WI-FI chip that need to unbind the Nand chip too.So,you need the Nand programmer to unbind WI-FI address or WI-FI address unlocked.If you have to know further this [https://www.youtube.com/watch?v=2aZsz6aF2gY&t=193s|may help] you.Have all process 7,7Plus and 8,8Plus have same process.