@BethHoward first point check that the supposed rogue device is not one of your own as the nsmes can be confusing and you may not recognise it or it nay simple show as unknown as the device is not returning a reply the hub can use.
If you are sure its not one of your own devices think about how it has been reconnected. Devices can only connect by using the password or by having physical access to the hub.
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65inch Sky Glass, 3 Sky Streaming Pucks, Sky Ultrafast + and Sky SR213(white Wifi Max hub) main Wifi from 3 TP-Link Deco M4 units in access point mode
