I use my A8SE II on a dual band router with separate 2.4 and 5g SSIDs, both of which have many connected devices. I'm not convinced that the dual bands are the problem. My guess is that there is a setting in some routers that causes the light to be disconnected occasionally, and switching routers is just a way of changing the default setting without knowing what it is. It's still possible the router is an issue, but it would not be my first guess, especially when your other light is operating correctly.
When you hold the center button on the light, does it eventually power up, going through each color channel sequentially? I would try it a few times. If nothing happens, try swapping power supplies with your other light.