I moved 2.4GHz and 5GHz to the same SSID a few days ago. If anything the problem got worse - I'm noticing my phone without a WiFi connection significantly more often than I did before.
I guess I call 2degrees. They'll probably want to reset it (not keen as it's configured for my setup), or replace it (I doubt it's a hardware issue), so it'll be a lot of hassle with no resolution. I guess I'll give it a go but maybe I end up buying another router.
I have the same ongoing issues too, have also modified the wifi Channel, it's so bad now that when I'm on my cellphone at home I turn off wifi as wifi calling constantly drops out.