Yup this is the same problem. I have a case open with ASUS right now. I updated them that even with 2 GT-AC5300 both running non beta firmware on AImesh 2 the problem still exists. So it’s not a beta firmware problem but you may be right that it’s an AImesh 2.0 problem with the Apple products. I would have to go back 3 firmware versions on the GT models to be before AI 2.0 And only 1 on the RT. How long has your system been up by rolling back the firmware? Sometimes mine will last up to a day or 2 then it goes downhill fast. I actually may return both of these junks because I am not willing to settle for AI 1.0, both (well now I have 3) of these routers are tri band. You can’t use the 3rd band 5ghz-2 with smart connect, even with wired backhaul with AI 1.0. You have to turn 5ghz-2 on manually and force it to workI have same problem on my router.
I'm using GT-AC5300(Main router), RT-AC5300(Node) and GT-AC2900(Node).
All the node devices connected under wireless and my Mac Pro has a problem same as you.
When the Mac pro connected to the RT-AC5300 with 5Ghz-1, there is no internet connection after using internet for one or two minute.
I have newest beta version firmware which is same as you.
My another devices working well, there are no problem with 5Ghz-1.
So I tried to change the Mac Pro connection from 5Ghz-1 to 5Ghz-2.
At that time, there was no issue.
Also I tried to find out what the problem is.
But I can't find it, so I just go back to the formal firmware.
Finally, my all problem is gone after using formal firmware.
So my RT-AC5300 router working under AiMesh 1.0, instead of 2.0.
If you are fixing the beta version issue, please give me a solution.
Thanks