I have seen a lot of routers that have problems with showing static up devices in their IP tables or attached device list. If the other devices can communicate with the xp8 (rk3) over Ethernet and the xp8 can serve up the weather and control IP devices then this is not your issue.

I have in the past had remote access stop working and generating a new system encryption key fixing it. This is done in system properties where you manage your RTIpanel licenses. I would also temporarily try putting in the actual WAN IP address instead of the dynDNS address in the system properties.

Have you tried deleting the app from phone and re-adding? You could also try changing the port. Lastly I would create a new system file from scratch and see if you can get that to work.