Devices connecting to new modem that are not suppose to.

Perhaps they are connecting via the MoCA function / feature ? MoCA is an alternate hardwired way to connect devices to a home network if ethernet cabling can not be used/run. It uses the existing coax cable wiring instead of ethernet cabling. 

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.

My pleasure ! Please circle back here and let us know how things turn out. Thanks !

Can you install the moca poe filter on the back of the modem even if you use the moca function on your network?

@user_xo1nyx 

Not if you are using the gateway's built-in MoCA feature / function.