R7000 Suddenly Won't Access Certain Sites

I have had an R7000 for a little over three years, and haven't had an issue until this past week.  With the new year, my work benefits reset, and I have been trying to login to the Express Scripts website for the past several days.  Before this past week I was able to access and login to the site like usual.  Now, however, I get am directed to a landing page with an error message "Something went wrong. We can't complete your request at this time."  I noticed there were a couple other sites that wouldn't load, either, but I'm most concerned about getting my prescriptions refilled!  

I initially thought it was machine-specific, and cleared the browser history, deleting cookies, clearned the DNS cache, restarted, etc., to no avail.  However, I've narrowed the problem down to the router, because I get the same landing page and message across machines (PC, tablet, phones), but when I turn off WiFi on my phone and access through mobile data, I'm able to access the site normally without issue.  

I've powercycled the router, I've powered down and powered back up the entire network and machines, but nothing is resolving the issue.  I tried using a different MAC address, resetting to factory defaults, etc., but nothing is working. 

Please help!

Editing to add firmware is up to date (V1.0.11.110), and this only happens for a couple/few sites; most are still working normally.