Devices on Wi-Fi that Aren’t Mine

I recently moved into an apartment building and started service with Xfinity. I only own ~4 devices that should be connected to my wifi: my laptop, phone, PlayStation, and TV. No one knows the password but me. However, when I open the Xfinity app and look at the devices connected to my Gateway, there’s at LEAST 15 devices connected at any given time that I don’t recognize. When I run a troubleshoot on any of the devices, it says that all of them have a “wired connection” to my gateway. I have no devices with wired connections. It’s pretty obvious that this means my neighbors are stealing my wifi, but how do I make it so they can’t? Xfinity support says that it’s impossible for anyone to connect to my wifi without the password but that’s clearly not the case.