I've got a 2925 and a couple of AP910c, and I don't seen any instability, or disconnects from my single iOS device (iPhone SE).

I did find with my 2925 that if I had WAN2 enabled, but nothing plugged in that sometimes that would cause the router to become massive unresponsive on wifi (but ok wired, iirc) - I reported that fault to Draytek and I've not experienced it in over a year now (before it would happen ever few days), but I have have numerous different firmware versions running on it since then.

I have observed a 2820 regularly crash & reboot when LAN2 again was enabled but nothing plugged in.

Thanks for your interest. I'm using WAN2 for vdsl via a 130 (concerned that the frequent reboots of the 2862, intentional or otherwise, were affecting my speed) and I use LAN2 for the Guest network, which has very infrequent use.

My issues seemed to centred around WAN2 being enabled, but the port not actually having a device plugged in/powered up. Sounds like that isn't the case, but thought I'd throw it into the mix!