Range Extender rpt v3 wont ignore BT Hotspot

Our extender had worked for months but recently lost the ability to connect to the net. It can still extend the network it "sees" but there appears to be no Internet connection from that source. 

After much searching I found that the problem is that all BT Infinity Routers have a public hotspot, so same device we extended from before, same signal strength etc.  So it appears that our extender is trying to extend the Hotspot and NOT the wifi network we usually use.

We can't find a way to ignore the hot spot signal. 

We also can access the mywifiext.net page because "there is no active Internet connection" - having had Sky routers before I am used to being able to connect to the router without an Internet connection!!?

So my plan is to get a network cable and wire into the extender in order to try and access the settings, but can anyone tell me if I do that then if and how we can block/ignore the hot spot signal. 

Any suggestions would be greatly accepted.  We can't access the router settings for long and boring reasons so isn't a viable option to try that. 

Thanks in anticipation