ESL point taken.
However, I think "getting 504 on different T-mobile devices when using 3/4G but not when on WiFi or using other carriers" is enough to describe the problem. If it the proxy that's returning a 504 there is not much that can be done besides checking apache logs and placing a call to OVH to make sure that all is kosher.
The first thing CSR should have tried was to try to open the domain in question on his T-Mobile device, no? Especially considering that this is not the first time that a domain is returning a 504 from T-Mobile network. No exactly an esoteric issue that requires domain side server logs to resolve. If CSR could open it then it was probably an issue with the client's APN config or something.