Me and neighbor connected to same network

Hi, me and a neighbor have noticed our internet slowing and dropping connection. Today without knowing, my neighbor approached me and asked if we were having issues. We both noticed when logging into xfinity app, that someone else( this being my neighbors devices) was logged into each other's wifi.I can see his devices connected to my router and he can see mine connected to his gateway. Thinking it was someone hacking in, I changed my wifi password  and restarted gateway prior to knowing this. I did some digging around and first thing I did was disabled Hotspot thinking that would fix the issue, it didn't. I read a few posts on here about people having similar issues and decided since his devices show up as moca and we don't use moca, I would just disable moca. I tried doing what was suggested in another post and unplug coaxial cable to gateway, then disable moca but it remains connected!!! This is very frustrating for both me and my neighbor. Either something isn't hooked up right on the hardware side or I'm missing something. I've never had so many issues with a router/modem, would rather bring my own gateway but found out I can only use select few!! Can we please have the internet we paid for!!?? We've been paying high prices only to find out we're not getting what was advertised! Please help.