You should start your own thread here.
The AC68U node did
not add successfully, yes?
The AC68U is end-of-life... it may eventually not 'mesh' with a router using current firmware.
Without knowing more about how you manage ASUSWRT firmware, and given the issue arose after some firmware updates, advice here may tend to be 'start over' to be sure the firmware is in order before troubleshooting further. In particular, install the firmware, reset it to its default settings, and then configure it from scratch... do not restore saved settings.
FW Reset FAQ
Reset button/webUI Restore/node removal clears settings in NVRAM; reboot restores fw defaults from CFE
Hard Reset via WPS button/webUI Restore+Initialize also clears data logged in /jffs partition
If you don't want to restart that much at first, try Hard Resetting the AC68U firmware and adding it again to the AiMesh.
OE