Just something else I have tried, since I want to stay on the 5 Ghz band only - the 2.4 Ghz band at Mum's house where the RE580D is installed is incredibly congested. I disabled DHCP server on the...

I installed this same model RE580D four days ago and have had precisely the same problem. For one person to have a problem might be dismissed as random, but for three to have the same issue suggests...