Can't access single website from ONLY this network

Hey all, I'm the network guy in the sense that I know when to stop and ask someone else... So this one is beyond me right now:

So, we can't access our company website (normal website, not an internal one.) from our network. Using cell phones' own data service will work fine, customers are purchasing orders, etc. I can ping the server IP from inside; but any attempt to load the site fails with a timeout. Ping and Traceroute from the gateway work fine.

I even went so far as to pull down the SDWAN that is managed by the ISP (No control AT ALL, and is just handing out DHCP for a single flat /24, I needed to go anyway) and replace it with a new Mikrotik router that I was configuring for another use.

We're stuck on Frontier, so getting them to look at this will be a nightmare, so I need to have any ammo I can to have them look into it. All machines can't access, and ONLY our machines; and ONLY this one site. I had our web team look at the logs, and they don't even see any access from our IP. Not sure where to go from here before I fight the good fight with our ISP. Any help with diagnostic advise would be appreciated!