I have a hAP ac², running the current long term version 6.43.14, but the issue existed in .13 too. Not sure about other versions because when I got the router, I upgraded it to the then current version .13.

The issue goes as follows: if a client that is somehow far away from the router connects to the 5GHz network, first everything runs fine for several minutes and then for whatever reason it loses connection and the only way to continue having connectivity is either to switch to the 2.4GHz one (manually or automatically after the client "realizes" that there is no connection) or to restart the router. By "loses connection" I mean, on the client it shows that it is connected to the router, although with less signal (e.g. on MacOS it starts with 3/4 or 4/4 and then drops to 2/4), but there is absolutely no connectivity to the internet or the router. This happens on different devices -- MacOS and Windows laptops, iOS tablets and phones, so it is unlikely that it is from the clients themselves. I understand that the 5GHz one has smaller coverage than 2.4, but it is not clear why everything works just fine for several minutes and then everything drops.

Any ideas what may be going on and how to fix it?

I am thinking about some firewall rule that is misbehaving and dropping connections. Something like a DDoS filter that sees the client as a ddoser and blocks it?
Just a wild guess. No info about your setup as none was provided.
Try /export from terminal and edit out what is not needed.

I don't think that it has something to do with the firewall as there are no different rules for the two networks. Also, I added logging to all drop rules and there were no entries to indicate that a client is misbehaving. If really needed, I would post the export here, but it would take me some time to remove the sensitive or not relevant entries.

What I have found out by running iStumbler is that the signal and respectively S/N ratio fluctuate a lot, say by 10dB and what seems to happen over time is that it hits some low level and then it "loses" connectivity, although still connected to the AP. I have read several discussions here about increasing or even reducing the power, reducing the width etc., but these didn't work reliably, e.g. reducing the width resulted in stable connection for some time, then it started misbehaving again.

Is there anything that I can realistically do about this besides setting some kind of repeater?