Try powering down the node. With the router up and running normally, hold the WPS button on the node while powering it up. Continue holding the button for many seconds until the LED quits doing its dance. I believe it comes on and off a couple times. Asus has an FAQ on their website which covers the process if you need further / better instruction.
Then power cycle the node. All this will "factory reset" it and when it first boots up anew in that state, I believe it will automatically configure itself as a node when it sees the router. It's not an instantaneous process, but I know on the XT8s if you want to configure one as an AP instead of a mesh node, better make damn sure it doesn't see another acting as a router after the reset, or you won't stand a chance.
Oh, beforehand, on the router, be sure to "remove" the node from any present "mesh" if the router thinks there's already such an association. Asus may have accounted for that situation, but perhaps not.