Have you tried doing a Hard Reset or even a full DFU Restore? IF you'r problem is software/iOS related, then one of these two options should help.
Otherwise, you are facing a hardware issue. The iPhone 6 Plus is prone to micro-flexing of the logic board due to its size and relative weakness of the housing. Many 6Plus have developed Touch Disease because of this. It's not unreasonable to assume that your phone has developed WLAN issues for the same reason. You also don't say much about the history of the device. Was it dropped or bent, did it come into contact with water or repaired recently? All of these things could be responsible for your problem.
IMO this is most likely a logic board issue and you should seek out a reputable micro-solderer in your area so that they can take a look at it ad do a full troubleshooting of the board.