I wonder if the problem could be to do with the DHCP lease management. What you may be seeing is the lease expiring after 24 hours and then renewing. You may like to try 2 things. Turn the router off and then unplug it for about 5 minutes. Then restart it and see if the problem persists in the next say 30 hours (may be 24 hours after the restart rather than at 21:30 or so). If you are still getting the problem and if you haven't played around with the router settings, do a hard restart of the router to reset the factory settings and then wait again. If the problem persists a call to the Plusnet helpdesk may help sort out the router settings.