When you are running in AP mode, the WAN port is just like the other LAN ports, no difference.
Since you are able to ping the routers, the networking looks good. Unfortunately, we can't get in to look at the syslogs to see what may be happening..but my guess is that the web server is down or not starting correctly.
My next step would be to try this....
Unplug each of your clients (not the powerline/switch interconnects) and then reboot the routers and the netgear switch. Replug you primary client you've been testing from and see if you can get to the gui's then. If you can, immediately go to the Administration > System page and enable telnet and apply (so maybe we can collect additional data later).
Then plug in each client one at a time and retest. As you do this, on the AC87, don't use LAN port 1, it has a special setup and I'd like to eliminate the most variables possible.
There was just another user where having an HP Printer plugged in was apparently causing the router web server to crash (still don't understand that one), so may want to save that one for last.
Ok, I will try this tomorrow....3.45am..time to go to bed
Thank you very much !