I’ve been trying to connect my devices to a bt WiFi connector(750) but when I put both wireless key and admin key it doesn’t work I’m connecting it from a Vodafone router
Have you paired the Extender 750 to your router using WPS? If so, your extender should then be given an SSID of "EXT-<router SSID>" and "EXT5-<router SSID>" which you should then be able to connect your devices to. When you do connect a device to the Extender and are prompted for the network key, this will be the network key of your router, not of the Extender 750.
I hope this helps.