(I'm assuming you are connecting the RT-AX89X directly to the Openreach ONT. If you are connecting it to an ISP router (e.g. a BT SmartHub), that's a different and additional set of complexity.)
I could be missing something, but you probably won't gain anything by using the 10G port for WAN. 1G Ethernet is full duplex, so can do 1000Mbit RX and 1000Mbit TX simultaneously, which seems like it would be sufficient for the BT Fibre 900 service (900Mbit RX and 110Mbit TX). A 10G port connected to a 1G port shouldn't have a significant performance benefit over just 1G to 1G; at the Ethernet level, both scenarios are identical (assuming the auto-negotiation works correctly).
If the default port setup for the router is sufficient for the service, I would stick with that, personally. ASUSWRT is pretty good as far as retail consumer router firmware goes, but it can get a bit feisty in less common configurations. You may well only have a 1G port on the Openreach ONT anyway, as it was only last year that they were talking about moving to a newer ONT with a 2.5G port (the original / previous Openreach FTTP ONTs were all 1G Ethernet, as far as I know).
Rebooting the ONT is certainly a good idea when connecting it to something else (even just a different port on the same router), when you encounter problems like this.
