@Tony The solution was to log into the web interface (the suggested URL didn't work, so I found the IP via my router and that worked) and reboot the whole system. Now it seems to be happy.

I have a 3-unit Deco E4 setup, which is working very well, at least in terms of WiFi throughout the house. However, having worked very well initially, the Deco app will no longer allow me to connect...