I have followed the wiki instructions for accessing my modem web UI on my AC66U's WAN port here:
https://github.com/RMerl/asuswrt-merlin/wiki/Access-modem-Web-UI-on-WAN-port-(no-script)
My modem's web UI was very slow to load up this way but it did appear eventually. Unfortunately, now I can't get online with any of my devices. The router shows that it is connected to the web through the modem but nothing connected to the router can get online. My modem is in bridge mode and my router is connecting to it using PPPOE. If I remove any of the changes from the wiki instructions, everything works except I can't access my modem web UI.
https://github.com/RMerl/asuswrt-merlin/wiki/Access-modem-Web-UI-on-WAN-port-(no-script)
My modem's web UI was very slow to load up this way but it did appear eventually. Unfortunately, now I can't get online with any of my devices. The router shows that it is connected to the web through the modem but nothing connected to the router can get online. My modem is in bridge mode and my router is connecting to it using PPPOE. If I remove any of the changes from the wiki instructions, everything works except I can't access my modem web UI.