Access Admin Panel - "Site Cannot Be Reached"

I can’t access the https:// address for the admin panel for my Pepwave Surf SOHO. I can’t access it via Chrome or Internet Explorer. It says “This site can’t be reached.”

Recently I’ve been losing internet access, and Optimum cable here in CT says it’s because of my router, not their modem. I’m trying to get to the admin panel of the router now to see if I can find out what’s the problem. I installed the router a couple years ago and haven’t thought much about it since. Has worked great. Now it’s failing apparently and I’m not sure what to do.

Can somebody help me figure out what to do?

Model # PepWave Surf SOHO MK3
Hardware version: 1*
My warranty expired 2019-02-11

Thank you in advance.