DIAGNOSIS The wireless backhaul (the dedicated connection between nodes) is struggling. Walls, distance, and interference from other electronics are the usual culprits.
FIX You need to optimize placement. 1) The nodes need a clearer "line of sight." Even moving them a few feet left or right, off the floor and onto a shelf, can make a big difference. 2) Avoid placing nodes near large metal objects, brick/chimneys, or appliances like microwaves and cordless phone bases. 3) In the ASUS Router app, go to the AI Mesh page. It often shows a signal strength meter. Use this as a guide while you slightly reposition the satellite node to get the best signal.
IF THAT DIDN'T WORK You can try forcing the system to use a different band for the backhaul. In the web interface, go to Network Map, click on your node, and look for "Connection" type. If it says "wireless," you can sometimes change the "Backhaul Connection Priority" in Wireless > Professional to prefer the 5GHz-2 band, which is usually more robust.
HEADS UP The XT8 uses one of its two 5GHz bands exclusively for backhaul communication. If the signal is weak, it downgrades the speed for your connected devices. Sometimes the best solution is adding a third node halfway between to act as a relay, even if you originally planned on just two.