This may help people who find that some or all of their devices are telling them their extender connection has no internet connection and opens a web page that shows the 'too many redirects' error.
Connect to your main BT hub (assuming that has no issues), open your browser and open web page 'bthomehub.home', this gives you access to your hub settings. Go into Advanced Settings (you will need the Hub Manager Admin Password from the settings card stored at the back of the Hub), select Home Network then Smart Setup. Set Enable Smart Setup to NO and apply.
This worked for me and I was very surprised how difficult it was to find this very simple solution on BT's help desk and that it isn't mentioned anywhere on the troubleshooting page for the Extender device itself, perhaps it should?