@DBedford 

Hello David,  

Not familiar with that particular router.  I haven't seen it in the US, though much of the DSL service here has transitioned to Fiber. There could be a regional setting that we just are not familiar with, though I would suspect it is more related to 2.4GHz interference or Range.   Since you have a plug that is working.  I would temporarily disconnect it and try to set up the plug with a concern in that location.  If it is still having issues then you may have a defective product.  At that point you would need to either contact our Global community that handled the UK/Great Britain region here:  The community this thread is on is actually for the US region exclusively so official product support for other regions is best effort only.  If this one test fails you can also reach out to UK support directly by emailing them at support.uk@tp-link.com