One of the possible causes is the auto software update of the router itself ! When a router is doing an auto update, it may need a restart to refresh its previous caching and linking.
However, In our observation, over 80% major source of causes are from router’s software bugs due to the bad programming in error handling mechanism and error prevention!
In these situations, users just need to restart their routers or unplug their routers from outlet that will resolve the problem.
Sometimes, an auto update may happen to your connected device like your cellphone that may impact on the WiFi connection so you may need to restart your phone as well. For example, if you update your iPhone version from 13.6.1 to 13.7 then you may need a restart.
Same solution applies to all other electrical devices.