Unable to join/ invite one particular player

A friend and I live in the same town and have the same ISP. Over the last few weeks our ISP has been rolling out new 844E gigacenter router/modems to all customers. He got the new one, no issues. Later I aquired the new one, we now both have the new router
and issues begin imidiately. All internet functions on all devices (PC, Ipad, phones, xbox, etc.) work flawlessly except for one single interaction; We are 100% incapable of joining eachother directly in any way, in any game, on Xbox one. If we are matchmaking
randomly or joining a mutual friend everything is fine. If it is just the two of us attempting to connect to eachother in-game in any way, absolute denial. PC gaming=yes. Xbox party=yes. All xbox games=no.

We have correctly powercycled all hardware (consoles and routers) including fully unplugging them, after and in between every step of every device setting change/ fix attempt.

Said attempts include:

•Toggling UPnP and trying to connect while on and off. The xbox IP was listed correctly with the correct ports being used while on.

•Disabling UPnP and instead port forwarding manually through a static IP using the ports listed for xbox one by Microsoft.

None of this yielded any results. I have read every forum I can find on this. There are some with the consensus that it is related to our ISP somehow denying direct connections between customers on their network and that this somehow only effects xbox. I
do not know what this interaction could even be a result of, or what to call it in order to contact my ISP and troubleshoot it. If anyone knows what this could be or believes it could instead be on our end, any input would be appreciated. I understand there
have been workarounds found for this type of issue, but they involve the use of VPNs or DMZ hosting. My thought is that UPnP is designed to manage device connections and ports automatically so this is never an issue. Additionally, if this was a design flaw
with Xbox live services or this router hardware, tens of thousands of people would be contacting these companies for support and it would have been fixed. This is an issue that effects just enough people to be annoying, but just few enough to be obscure and
impossible to find a reliable solution to. Thank you for your time, please for the love of god save me.

:)