Well I appear to have solved my phone disconnection problem but I suspect you won't like the solution....

I have been setting up a mesh of 3xAP903s and noticed several things - although the 3.9.3 firmware on the 2926 supposedly enables it to act as the mesh root, in that mode it resolutely failed to detect any of the AP903s. Hence I turned off the mesh setting and left the router as it had been before - i.e. as a 2.4 and 5 GHz "access point". (Bizarrely however the Draytek Wireless app on my iPad still seems to think that the 2926 is a mesh root, even with that setting disabled, but that's by the by..)

I then noticed that wifi devices - my iPhone in particular - appeared to be quite happy to connect to the AP903 mesh a couple of rooms away, but wanted nothing to do with the 2926 which was 3 feet away.

(All of these devices were running the same 3 SSIDs on both bands). So I thought, well if nothing wants to connect to the router why should I bother with it? I turned off all "my" SSIDs on the router but left a single main (different) SSID active on 5GHz - so I could still connect to the Android phone hotspot.

That connection has now been up and running for 2 days with no disconnects....go figure, as they say.

Sounds as if there is a big in the firmware! Found a solution also! - the router was seeing the phone as WAN2 but not triggering a connection/connecting to the phone's 4g. But what I found was that a laptop would happily connect to the phone's Mobile Hotspot and connect to the internet over 4g:-) … and strangely after there was that data connection already using the hotspot that was sufficient for the 2860ac to then also connect to the hotspot!!, and it remained connected after the laptop was disconnected from the hotspot!
Got to test a bit more, but it is as if the 2860ac can connect to the hotspot but is unable to "activate" the hotspot's 4g connection?...