c3700 connection issue - only able to access a few websites

I don't know a whole lot about routers and web connectivity issues so please bear with me. I've been following another discussion on this forum that appeared similar to my own, but the "solution" didn't work for me. That particular discussion was https://community.netgear.com/t5/Cable-Modems-Routers/c3700-connection-issue-only-able-to-access-a-few-websites/td-p/971632/page/2

The problem: inexplicably, my C3700 router will not connect to certain websites (like piazza.com, speedtest.net, and others, and virtually anything that has a forum component it seems). I moved to a new residence, but my ISP remains the same (Comcast), and this issue didn't exist several months ago.

I've had Comcast reset my modem and the connection almost a dozen times, and I'm pretty sure it isn't them anymore. I've done a factory reset, changed my DNS addresses to google's (8.8.8.8 and the secondary as 8.8.4.4), and turned off IPv6 Firewall Protection, but nothing has worked.

Help would be greatly appreciated since I'm starting to lose my mind figuring out what the deal is!