Can't stay connected to multiplayer servers

I have recently moved to my parent's place in Germany from the UK, trying to set up my Xbox One X. Can connect fine to Xbox Live (on a wired connection) and when I run network and multiplayer tests they are both fine, the NAT type is also open. But can't
stay connected to any online servers whatsoever - get booted off EA servers for FIFA or Activision servers for CoD: MW almost instantly when I log in. I had no problems initially but now this is constant and won't resolve itself. The router is a Genexis Platinum
7840, provider is Deutsche Glasfaser.

I have:

• Changed my Xbox system location to Germany as well as Microsoft account location
• Enabled UPnP on my router settings
• Tried port forwarding based off the recommended ports from Microsoft
• Disabled UPnP and port forwarding to enable a perimeter network (DMZ) using my Xbox's IP address
• Reset router to factory settings and rebooted (re-enabling DMZ)

None of this has worked! Anyone know what to try next?

:)