I can See and Access Someone Else's Network in my Apartment Building

I got a modem to set up my network for the first time after moving into a new apartment building, and as soon as I set up the modem, my Xfi app told me that 10-15 devices that I did not recognize immediately connected to my network. After speaking to support for many hours across 3 days, the resolution to this issue was that I could just see these devices on my network, however they weren't actually connected. This in itself is a massive security risk, as I had access to the make, model, year, operating system and IP address of the devices of someone else in my apartment building, and I assume that they had mine as well.

Fast forward one month, for some reason the internet on my desktop was no longer working and anytime I went to a webpage it said that my device was paused. I checked in my Xfi app, and it said that my desktop was indeed not paused and the troubleshooter reported no internet issues. Again, I contacted support but they were not able to fix the problem, insisting that it was an issue with the network card on my desktop (even though I tested my connection using hot spots and another Xfinity network, and had no issues). Throughout this process they recommended that I went to 10.0.0.1 to look into the details more closely, however for some reason my credentials never worked for logging in. A representative provided me with the admin login, and to my surprise after logging in all I could see was the details of a network that was not mine. I assume that this network is the one that belongs to the person in my building who's devices I can see, based on the name of the network/password and the device names I had seen earlier. After reloading this page a couple of times, I noticed that 10.0.0.1 would randomly switch between showing me my account details and the details of the other person in my building. I went to perform a factory reset after seeing this, however in doing so I accidentally reset the modem of the other person (as I did this through 10.0.0.1 and again I definitely shouldn't be able to do this). Funny enough, this actually resolved the issues with my desktop, leading me to believe that the other person in my building had paused my device on their Xfi app, which was messing up my internet connection without my own Xfi app being able to see this. I tried to contact support to get someone into the building and take a look at this, to which they told me that it could cost $100 to fix an issue that is very clearly with Xfinity. I would love for this issue to be resolved, as I've already drained many of my own hours in chats/calls/troubleshooting with many different people.