Occasionally and regularly I cannot connect to some of my devices on my LAN from within my network.  If I reboot my router, then I can connect to anything inside of my network, but usually about a day later, same issue.

I'm using chrome, but tried Edge.  Again, rebooting the router always is a temp fix (for a day or so).

For example I have a IP based power switch on my LAN and I even have an external port forwarded in case I'm out of the house and want to connect to it (via dynamic dns).  At home, I cannot connect to either the inside IP or outside address (until I reboot my modem).

I should add I'm using hard IP addresses inside the network.  I've tried these connections on 3 other networked computes and 1 sandboxed Windows session (bare bones), so it makes me think there is something goofy with the modem. 

This also happens when I try to connect directy to the router (local or external).  However, I can connect via Tether!

I don't recall making any changes to my modem lately.  Based on the issue above and rebooting the the temporary "fix" where should I look in the router settings?