unknown device attached to gateway

An Google Android 9 is attached to my gateway. This is not a device anyone in my home uses. I live in a rural area so only a few homes within the range of service. I have not shared my password. How is this happening?  How is this device attaching to my gateway?  i have disabled and will change my password but just trying to determine if someone has hacked or is their a burner phone in my home ?  Thoughts ?