Double NAT Issue for Xbox One - Single Router

Just set up the Netgear X4S AC2600 to our fibre connection here in New Zealand (Chorus), white fibre ONT box us running the lan cable into the back of the Nighthawk, which is then connected to the xbox via wireless.

Getting a double NAT issue on the xbox, but all solutions I found online don't seem to resolve it (setting up AP point/bridge mode), as the router bridges to nothing due to there not being a second modem in line (internet coming straight from fibre ONT box).

Tried port forwarding, tried opening NAT on the router. Tried setting up DMZ. Any other suggestions?