I recently switched to virgin media from Plusnet and experienced constant disconnection on my Xbox. I setup port forwarding to the following ports to fix this. I got the list of ports for the port forwarding from the Xbox website.
80 - TCP and 88, 500, 3544, 4500 - UDP and 3074 - Both. Also setup by default on the router were 2075, 2075, 1200 - UDP.
This ultimately fixed the constant disconnection on the Xbox, however when playing Call of Duty Modern Warfare Warzone I experience very high lag and lots of rubber banding. I did not experience this when I was with Plusnet but have only experienced it since switching to Virgin Media. On my Plusnet router I had setup port forwarding to Xbox Live which was a port forwarding option so I didn't have to individually select ports but just select Xbox Live under port forwarding.
I am wondering if anyone else has experienced this and has a fix or if there are other ports I could add to the port forwarding to fix this.