SB8200 Website blocking?

I actually encountered the same issue two weeks ago, a sports forum I visited daily suddenly can’t be reached, I first thought the website was down but it lasted for a couple of days,  I checked isitdownrigtnow and there was no report about the site was offline from other people, it seemed it was just me. So I used my phone and I turned off the WiFi, and I found I can visit the site with 5G connection from T-Mobile. It looked like the issue was from my Xfinity internet at home. I called Xfinity and I explained the issue with that website, and we did a lot test and reset, but nothing had changed, I still can’t visit that particular website for some reason. Then their tech support asked me whether there is XFINITYWIFI hotspot nearby, I told them there is one, the signal is not very strong but I can connect to the hotspot from my home, so we did that, connected to XFINITYWIFI hotspot instead of my home WiFi, and guess what, I can visit that site instantly with no issues whatsoever…we never really figured out the root cause for the issue,  but I knew the issue was not XFINITY, they didn’t block the site, and the issue was not from my Wifi router, I used the same router and didn't’ change anything,  and I never thought about the issue could be caused by cable modem here since I have been using it for more than a year and there was no issue before. 

But after read your post, now I think the issue was from the Surfboard cable modem, mine is also SB8200 model like yours, two weeks ago, I had to connect to XFINITYWIFI hotspot (bypass my own cable modem SB8200) if I want to visit that website. Last week, my SB8200 stopped working, I am currently using a different brand cable modem and I have no problem visiting that website with my XFINITY home internet. 

SB8200 is a simple cable modem and there is no specific configuration in it. Could you connect the modem directly to the computer through ethernet cable and trying to access it.