More than a year has passed, ASUS has released several firmware updates for different mesh-devices, but has not bothered to fix this security issue. That's all You need to know about this company.
Personally, I was able to defeat this firmware only in this way: update via recovery mode, settings reset, complete router reconfiguration. Positive aspect: everything works fine; negative: update took a long time.
It was the same with me. The router seemed to be trying to protect me from this firmware, but I didn't listen to it and got what I got. Don't repeat my mistake — skip this version.