Followed this advice and it's working nicely, so thank you. Hoping that the doubt NAT issue never rears it's head.That is how it works, you don't need modem mode, it just means you will have two dhcp servers, one in the sky hub that only serves the AX3, but you must only use the sky hub for that device, disable wifi on sky hub and don'y plug any other devices to it unless you want two separate networks.
This method worked fine for me.
Main router + 2 more mesh nodes covers the house with minimum 20Mbps and my actual wired speed at the wall is 37Mbps, so I'm happy.
Now just need to get a wired connection over to the external home office...