Two days ago I bought the RBD53iG-5HacD2HnD and transferred the necessary configuration from my old RB951 (DHCP, Wireless Access list). I reconfigured the WIFI network. I connected the Router instead and after several minutes I noticed the problem. Namely, the 5GHZ network comes and goes! One time it can be visible for 1 hour another time 5 minutes! Devices connected to it lose connection with the Internet, and the phone switches to the 2.4GHz network!
5GHz network is set Mode: ap Bridge, Band: 5GHz-A / N / AC, Chanel Width: 20/40 / 80Mzh XXXX, Frequency: auto What could it be? Where could the problem be?

Most likely the 5GHz issue is caused by DFS -
https://en.wikipedia.org/wiki/Dynamic_f ... _selection
This can be avoided by manual frequency selection on a non DFS frequency. A "Scan List" in wireless settings can also assist.

I always set WiFI channels to manual mode and choose the least loaded channel. This was the case in the old Mikrotik, and it is also the case with 5GHz WiFi.
Unless it's about some other setting than manual channel selection?

I always set WiFI channels to manual mode ...

Your first quote says otherwise

Mode: ap Bridge, Band: 5GHz-A / N / AC, Chanel Width: 20/40 / 80Mzh XXXX, Frequency:auto

after upgrade from hap ac2 to hap ac3 speed of wifi is faster, but coverage is same, signal after 2 walls same as hap ac2... though would be better