Wifi , has device connected which is not mine

First. Is this a Comcast rented gateway device ? If so, check to see if the MoCA feature in it is enabled. If it is, and if you do not have a POE (Point Of Entry) MoCA filter installed on the coax line, yours and your neighbor's gateway devices will be able to connect with each other like one big network. Disable it if you aren't using it. If you are, install a POE MoCA filter on the input port of the first splitter off of the street drop to keep the signal in your premises and to keep the neighbor's signals out. Your neighbors should also have POE filters installed.

To disable MoCA on your device go to the gateway login http://10.0.0.1  

Default login info is:
Username: admin (all lowercase)
Password: password

Once in you will find the drop-down on the left of the screen that is labeled “Connections”.

Click the drop down---> click MoCA---> right side of screen click “disable”---> save changes.

Bear in mind that we've seen many posts here that the MoCA feature gets turned back on with their overnight re-boots / updates for the device. The best policy is to be sure to use that POE filter ! Amazon has them: https://www.amazon.com/SNLP-1GCW-Filter-Eliminate-Multi-Room-Interference/dp/B07SLD9QPH   

Or you can get one for free at your local Comcast / Xfinity store. 

If you are not using the MoCA feature on your home network, you can put the POE filter right at the back of your gateway device instead.

Hi, @user_f7cb3j! We just wanted to check back in with you today to see how is everything working.