I bought a BT wifi extender on amazon as I have a very thick wall between the router in the lounge and my flat's bedroom and even radio streaming buffers in the bedroom.
It's not working. (I have set it up according to BT's instructions, with ethernet cable to HH4 etc). It won't find - or doesn't improve - my main BT network. Still buffering.
I tried connecting the iPad to the extender's own network but it refuses to recognise the network key/password printed on the back of the unit.

Mini wifi extender 500 is the product. Got good reviews on amazon. Not sure if I should change it for another one or try to get hold of BT support ().

I sounds as though you are not actually going via the extender but rather via your existing WiFi network. That is to say the extender may as well not be there.

The way that these boxes work is to add a second wifi network to your dwelling so you MUST connect to that network in order to benefit from the extender.