I thought my XFi home network was secure but it’s not!

Security on my apt home XFi network seems to be an illusion not a reality. In the last week, about a half dozen unknown people have jumped on my home network. Do I need to call Comcast & change my network name? I have it password protected but they still are getting on somehow. I can pause their unknown devices indefinitely to discourage their ability to log on & use their devices on my home network but again, if they shut off their WiFi & use mobile data turned on, then they can easily bypass my best efforts to keep them off my supposedly “secure” home network. What’s the answer to fix this problem?