The problem: Connected devices on my Hub One, when their profile was deleted by me on the router's management UI, would never be able to get an IP again over WiFi. Other devices connected just fine. No amount of reconnection attempts would result in an IP for these "deleted devices". It seems the router software design is intended that deleted devices should never be able to reconnect again, ie each becomes a "forbidden device". It's just not documented anywhere. Users have to find out the hard way.
Solution: reset your router, using the recessed button on the rear. Was suggested by a PlusNet support agent, who said he was not aware of the issue. You naturally end up losing all your configuration changes, but at least your WAN (broadband) credentials are saved and reused, it seems, as they do not need to be reentered. Then, when the reset router reboots all those banished devices can reconnect and get internet access.