@AndyT531 

Hi, Archer AX6000 is compatible with Startlink internet and for physical connection or setup, please refer to: How to Connect and Set up a TP-Link Router (or Third-party Router) to Starlink Internet

For the range extender, AX3000 is not the model no. actually, RE705X, RE715X etc. are all AX3000 product, and they are both compatible with the Archer AX6000 router.

In addition, since you mentioned that the building is with very thick walls, thus the WiFi signal will be reduced largely through walls, it would be better to use wired connection to extend the WiFi range if wired Ethernet cables are deployed inside the walls, in that case, it is recommended to consider EasyMesh routers which supports Ethernet Backhaul: EasyMesh Routers Support Building a EasyMesh Network via Ethernet Backhaul NOW