Particular user can not access site.

the guy in Atlanta can't get to it through his WiFi

What message/behavior does he experience when he "can't get to it"?

If not an explicit "site is blocked" message, has he tried doing a traceroute to the site from both connections to see the difference?

He is getting the standard site not found connection message.  Had not thought about the traceroute, thanks.

Hey @akmiecik. Thanks for reaching out to us, and we'd be happy to help with your website not being accessible from a different location. 

We appreciate @JefferMC for providing that helpful information on the next steps to take. 

Please let us know the results after they've run a traceroute to the site from both connections. Also, have them try clearing the cache and cookies from the device they're using. There could be saved data that is causing them to be unable to reach the website. 

We look forward to hearing from you. 

Robert, AT&T Community Specialist

His trace route gets a Requested Time out after 107.212.168.228

Does that mean ATT is blocking it at that level?

Is this his traceroute?  There's nothing wrong with this traceroute.  One hop in the middle that doesn't respond isn't at all a big deal.  This shows that AT&T is delivering the this test traffic to the peering provider and it is being carried all the way to the destination.

My traceroute joins his (i.e. uses identical hops) starting at 4.68.62.255, and I get to the website without an issue.

So, this is a new one for you too.  I don't feel so bad.

Given that he can see it on his phone when using cellular, but can't on WiFi, is there something in the router (BGW210) that could be holding it up?
Or does the traceroute rule that out?

If the traceroute is fine, does that me AT&T is not bocking it in anyway?

He can not get to any websites on that server.  Which means for some reason he is blocked from that IP address?

(yet, the traceroute worked)  Befuddling.

Yes, it appears that that particular server, or the network equipment that feeds it is dropping traffic from his IP.  Either it's configured wrongly, or the IP address is in a list of previous bad actors (or close enough to be caught by a large block).  The owner/managers of that server would have to look into it.

I am server guy.  It's blacklisted on two of the many blacklists out there.  I guess we can say ATT uses those two particular lists?

I could not get into his router so I had him whitelist it in Windows Defender and that worked.

Still have a problem on his phone where javascript will not run on that site on wifi but will on cellular.  He can get to the site on phone but the scripts don't run.

Thank you Jeffer for the help.