What are you streaming Netflix on?
If it is a laptop or PC, can you put it next to the router and see if you get the same problem?
Not sure if this will help but the BT 600 (similar) was a problem for me. I assumed that if all the lights were on it would be connected properly but this was not so.
When you set up the broadband extender did you plug them in close together and link them? If not your extension could have set up it's own hot spot and you need to use it's password to attach to it. A LAN will function fine through it but wireless will be a problem. If you linked them together then the extender should be just a replica of your hub. You should see it listed in available networks on your phone as BT Homespot but shouldn't need to connect to it with a different password/key.
It also depends on the age of your house wiring whether these work successfully. If you are in an older property and you have separate power ring mains, this extender will probably not work.
All this was in the small print at the bottom of the instructions in the box and I hadn't noticed it until a friend who had the same problem told me what to do. Now they are all working properly, connectivity is brilliant. If only I had consistent internet provision to my hub I would be in Utopia!!
I think the helpdesk is getting confused between a wireless repeater, and a wireless access point (hotspot) provided by the BT white powerline essentials 500.
Do you have any other powerline type device connected to the home hub?