Changing an Alexa device from 2.4G ti 5G, is bad advice because the range of 2.4G is better than 5G and better connection success is with 2.4G. ALL my many devices are on 2,4G, so if you know for sure your device is on 2.4G stay there.
Instead, try this. unplug the device for FIVE minutes to allow the device to completely clear its memory cache. After five minutes is over, plug it back in. Let it fully boot and then try things again.