What all have you tried? This is what I had to do several months ago to get one back online:
The main keys for me were
disabling 5 GHz Wi-Fi so that my phone was forced to connect via 2.4 GHz because no other network connection was available
taking the Plug to the room where the gateway is, because that was going to be the only available Wi-Fi in the house, and I wanted the signal to be strong
After I got 'er reconnected, I re-enabled the 5 GHz radios on the gateway and my WAP.