Sounds like you are using the ISP modem as your WiFi router. A better networking experience can be had if you get your own decent WiFi router. There is much to gain, from features, security, performance, and ease of network management. But don't cheap out with cheap router. It doesn't need to be expensive. I like to recommend the Asus RT-AC68U as a medium priced and very capable router. Then you tell the ISP company to reconfigure your modem to just be a modem by having them put it in bridge mode which turns off its mediocre firewall and WiFi capabilities and becomes just a modem as you won't need that since you'll be sporting a new router.
As for your issue it does sound weird. I would power cycle the modem/router one more time. This time keep the power out for at least 30 seconds. Then power it back on. Wait about 3 minutes. Then attempt to reconnect devices. Telling devices to forget the WiFi password and rejoining is also a good idea. You should also power cycle these devices.
If all this doesn't help then call the ISP and have them factory reset the modem/router.
