No, the ATEMODE tweak on the RT-AC66U B1 only enabled it to connect as a node with wireless backhaul - it wouldn't connect as a node at all before that fix.
After I got that working, I tried to enable wired backhaul and was greeted with the following message:
"* The following node(s) do not fully support this feature temporarily. RT-AC66U B1 (Home) Your configurations might not work as expected. Please regularly check your firmware version of the node(s) are up-to-date. "
I have been hoping for further news in this thread and the release notes of later RCs ever since (hence my excitement at your previous comment!) I haven't commenced the RC2-8 update yet... sounds like I should stand down for now. If I am lucky
@ASUSWRT_2020 is listening in!