Xbox One Strict NAT using WiFi extender

I bought an EX2700 WiFI extender a couple of months ago to use with my XBox. Up until recently, it's worked great, but today when I switched on to play games I got a warning that it had a Strict NAT type, meaning I'd have trouble connecting and playing games.

This doesn't affect video streaming, so far as I can tell, just gaming.

I've tried both wireless and wired connection to the extender, but no joy with either setup. I specifically bought this extender because I wanted to play games, and the distance between my router (ground floor, back of house) and console (first floor, front of house) meant that a wired connection between the two was impractical.

I've looked into port forwarding, which I know is the usual solution to this issue, but I have no idea if I should be doing this for the XBox on the router, the extender on the router, the Xbox on the extender...

All help gratefully received - I've been trying for an hour and a half and have completely wasted my Saturday morning getting nowhere!