Router WiFi

I have a Cox modem/router. It provides 2 separate WiFi options: 2.4 GHZ and 5.0 GHZ. I prefer to keep my iPhone on the 2.4 GHZ option as it's stronger but with less range. I've been noticing that the modem indicator light associated with the 2.4 occasionally blinks. When I turn on the iPhone - and if I check - it automatically selects the 5.0 option. I can easily change back to 2.4, but why would the modem indicator light occasionally blink and the iPhone select the 5.0? We live in a one story condominium so it cannot be that the iPhone is searching for a stronger signal; the 4.5 is the stronger of the two. I've unplugged/replugged the modem but after several days it still shows a occasionally blinking modem indicator light. Any ideas?