Strange Issue - Any Ideas?

I've had a recurring issue where both wifi and wireless stop working. Well, the wifi (2.4Ghz and 5Ghz) networks are still present, but either there isn't any internet for devices already connected, or if I try to disconnect/reconnect to wifi, I get "incorrect password" or "something has changed in the wireless configuration, please re-enter the password".
As soon as the modem/router is in this state, power cycling or resetting the modem to factory settings solves the issue, for between 5 and 30 minutes, and then everything stops working again.
Adding my own personal router to replace the Fizz built-in router did not solve the issue.
I strongly believe is it related to the number of devices we have, or a specific device causing issues. We have about 20 devices in total: iPhones, iPads, laptops, Google Homes, Google Nest fire alarms, a few intelligent light bulbs, alarm system... I've found that the issue is not present if I purposefully keep the majority of the devices disconnected. One interesting note: when the issue occurs, my personal router's configuration page says it's a DHCP issue with the modem and to contact the ISP to troubleshoot.
I talked to Fizz Support 3 times in a row and even had a technician come over to check things out. Because it's an intermittent issue, none of the typical support steps have helped. They always say that everything seems fine on their side of things.
Any ideas on what could be causing it? Is there a known limit of number of devices that can be connected at once?
Thank you!


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