Unknown Attached Device

I have a router/modem combo and I’ve reset the admin generic pw of “password”, I’ve changed the name of the router and only connected my phone to the router. When I go to the attached devices section I see two devices attached. One is my phone and the other is unknown. It doesn’t show a IP address and it has a MAC address. Nothing else should be attached. I think someone has hacked into my router and attached their device? I’ve reset the router and changed all the passwords 5 times and it always comes back. Does anyone know how to get rid of this attached device?

Thanks