@Arejoe 

To extend your wireless coverage in your premises just set the second AX80 in AP mode (Case 1) and set its wireless SSID the same as the first AX80.

Thus you'll not introduce a double NAT in your LAN and both routers will have the same SSIDs.

Ethernet connection should be: first AX80 LAN port => second AX80 WAN port.

If AX80 supports EasyMesh feature, you can use this video guide for configuring a wireless mesh network between those two - thus you'll have seamless roaming when moving around your premises.