I cannot access 192.168.100.1 but I am still connected to internet

I keep getting website is unreachable.

When I reboot, I am able to access this again. Hours later it is unreachable again. i never lose internet access.

Only way I am able to access is to power cycle modem but that clears my logs, which defeats the whole purpose of me trying to access the UI in the first place.

Why is this happening?? How do I stop/fix it? Is it a router setting? an ISP problem?