Actually the option for both backhaul and fronthaul is for 5 GHz-2 only. Screenshot of app setting. The only option for 6 GHz is Enable Radio, (at least with SmartConnect on maybe)...
Here are my settings. In order to set-up your backhaul using the 2.5gbps ports you need to factory reset your aimesh node then connect the router to the node using the 2.5gbps ports on both devices (do not use the WAN port).
The instructions from ASUS show going from and Ethernet port on the Primary router to the WAN port on the AiMesh node.