WiFi network keeps disappearing

I’ve had two different modems in the last week (an X7b and now an X8b) that I’ve setup and created wireless networks with. With both modems, the wifi network appears for a day or two and everything connects to it beautifully. Then, inexplicably, it disappears and no device can locate the network. Anything that is physically plugged into the LAN ports connects to the internet without problem. Once the network has disappeared, it is impossible to get it back. I’ve rebooted and restored the modem to factory defaults. I’ve changed the network name and password. I’ve contacted Xfinity customer support and they directed me to get another modem, but I just can’t believe that this is a modem problem. Any suggestion for what I can do to solve this problem? Has anyone else experienced this?