My non-xfinity router changed the subnet IP address to avoid conflict with my modem

I am considering my self tech savvy but I faced a really weird challenge today with my home network. The internet access started to fade away after midnight and I thought it was an outage in the area or there was some equipment work on the Xfinity side. Today around noon it happened again and eventually we were out of internet. This is kind of a disaster for a family like mine, two retirees without internet and on top of that two home phone lines are VOIP, my cell phone maxed out the data and what not. The first thing I called tech support to find out if there was an internet outage in my area. Instead of telling me there was none before anything, the guy started to preach me about restarting my modem and that my modem was not compatible with the new technology and what not. Eventually he tried to sell me a AT&T internet service which is not available in our area anyway. To make the story short, my internet service was recovered and I got my laptop and phones working fine. However my wife could not get any internet even though her desktop was connected to the router by an ethernet cable. It was really a puzzle for me what is happening.  I looked on my wi-fi specs when I realized that my IP subnet address was changed. From the old 192.168.1.1 my router Gateway's IP address became 10.0.0.1. When I logged to it's home page I got a message from the router that my IP was changed to avoid conflict with my modem which is a relatively new Arris DOCSIS 3.1 model. This has caused all devices on my home network that have fixed (non DHCP) address to stop working, including earlier mentioned my wife's desktop. I am still wondering how and why would Xfinity change my local network subnet when my wi-fi Netgear router is not an Xfinity device (bought and owned).