Can’t sign into Xbox live for backwards compatible 360 games on Series X

Hi. So this is a difficult question for me to ask as I am not as tech savvy as others but I will try my hardest.

I recently noticed that while playing Fallout New Vegas as a backwards compatible game on Series X, that I was receiving no achievements. I just put this down as an oddity, and moved on. However when I attempted to play Gears of War 2 online with my brother, it wouldn’t allow me to, saying I needed Xbox Live to do so.

It made me test my internet connection through the Series X network screen, which came back successfully. However, the 360 emulator for backwards compatibility refuses to acknowledge this.

As it is, my profile has been downloaded and signed in for the 360 games, it just doesn’t recognise that I have Xbox Live (which I do).

I have attempted hard resets, changing my DNS numbers, clearing my Xbox 360 cache (which deleted all my 360 saves - RIP) but nothing has rectified the problem.

A few other things to note. I am using wifi not Ethernet cable. I have an open NAT. Everything else through my Series X works perfectly. The 360 Xbox live used to work at my old house (a year ago) and stopped working after the move and upon setting up a new internet connection. I am able to play Xbox live for 360 games using a mobile hotspot (although I won’t do this for data reasons).

It makes me think there is something my ISP has done but I am not technologically clever enough to know what.

I know this is a very specific issue. I’ll be grateful for any assistance.

Edit - The error code that comes up through the 360 games is 8015190E.

:)