PENDING Remote site unable to communicate with SUP for monthly patches

[KeyboardHoarder]

For context, my environment has about 300 endpoints in total, A primary site server that acts as the MP, SUP, it's local DP, etc., and 4 other remote sites that all have their own site servers with DP roles and each site also having its boundary group set to its AD site. My environment is entirely in the US except for one remote location in the UK. It's an acquired location that had our services stood up earlier this year and we've been working away and getting everything up to speed. This UK site is where my problem exists, none of the endpoints seem to be able to communicate to the SUP that is the primary site in order to receive it's monthly patches. WUAhandler.log reports a 0x80240440 error everytime an update evaluation is run. In the screenshot attached, while monitoring this live, the "Async searching of updates using WUAgent started" line will sit for about 5 seconds then spits out the failure. Our SUP uses port 8530 so no SSL certificate shenanigans, I've disabled the windows firewall on both sides to test and make sure it's nothing with that, and the network engineer confirms that it is no block coming from either edge firewalls. Software distribution to the DP works, clients can install from software center and can also PXE just fine. The other US-based remote sites can communicate just fine. Any recommendations?

 

[KeyboardHoarder]

Some further testing done. Testing this powershell line on a client at the remote site:
"Test-NetConnection -ComputerName <WSUS_Server_Name_or_IP> -Port 8530"
succeeds. Running this powershell line:
"Invoke-WebRequest -Uri "http://<SUP_Server>:8530/ClientWebService/client.asmx"
Results in this response:
"The underlying connection was closed: An unexpected error occurred on a receive."
Feels like a network block of sorts or something misconfigured with the way SCCM handles WSUS requests. I'm just not knowledgeable enough to tell what.