Yes, need to reset everything using the WPS button, otherwise I read that they will sync bad settings back.
I turned off secondary nodes, reset the main node first, then reset each secondary node.
[Wireless Router] ASUS router Hard Factory Reset - Method 2 | Official Support | ASUS Global
I...
For anyone else wrestling with this, I finally fixed it by doing a hard reset of all nodes.
Tried every combo of wireless settings imaginable but Asus seems to have a bug that corrupts settings. Besides the Nests going offline regularly there were other strange symptoms like the 5Ghz channels...