Internet connecting to specific websites and not others

I'm having connection issues with my Xfinity internet on my Mac.

When connected to my router, I can't access most websites, but can get to others.

For example, I can access Facebook, or fast.com (which confirmed that I do have internet access), but when I try to access reddit, Safari tells me that I'm not connected to the internet and that "the server can't be reached".

This only happens on my internet. I tried it with two routers and the same thing happened, so I don't think it's the router. I also connected to a different wifi network (xfinity wifi, from the exact same location) and I could connect to all the sites that weren't working previously.

I've tried clearing my safari cache. I've cleared my DNS cache. I tried connecting without plugins or javascript. Nothing made a difference.

Any ideas?