I just ran into this same issue running 3004.388.10.2 of Merlin's firmware on my AX88U. Following the suggestion to use the app I typed in the password I wanted to use but the app flagged consecutive characters. I fixed those and voila! I'm wondering if in the web interface if it's possible...
I recently switched from Xfinity to ATT fiber and my network performance has gone to hell. I put the fiber gateway into passthru and assigned the mac of the router and that much seems to work. I can web into my Blue Iris and home automation systems from outside just fine. Internally...