I'm using a Xiaomi Router 3g with OpenWRT (with latest fw).
I don't explain what happens.
I'm using 2 wireless (2,4+5) with same SSID, one with wifi N, the other is wifi ac.
After a period when I want to play music on my iPhone and want to stream to an "Airplay 2" devices : 1 HomePod and 1 Airport Express (latest fw with Airplay 2 compatibility), sometimes, all Airplay 2 devices disparear.
If I reboot the router devices appear again.
I thought maybe the Airplay devices were on a separate band so the issue would be with the band not the devices in particular.
Could you give it a try to choose a channel rather than leaving it to auto? Maybe you have interference where you live, the router keeps changing channels and the Airplay devices don't like it?
There is no "default". For 2.4 GHz, Channels 1, 6 and 11 are the preferred as they are not overalpping. It could be a good idea to run a WiFi utility to see what channel is the clearest, especially if you live in a building with many neighbours. Though this is not always helpful, as often you will have the neighbours having their channels set to auto.
For 5 GHz, you are less likely to get much interference form neighbours as it has shorter range in general.
For the sake of trying, you could just turn the 5 GHz off and have everything on the 2.4 GHz to see if that works first before you look into the 5 GHz.