I think I solved the problem. If I am correct, the problem is on the Fire Stick side. I had connected it to a guest network on another router I was testing, using the same SSID as is on my ASUS. That test router had better penetration on the 5g band than the asus (it has beam forming, though I don't know if that had any effect). When I switched the stick back to the ASUS router, the problem began. After doing a little Googling about the Fire Stick connection problems, I found that it doesn't forget networks*. Being that I used the same SSID on both routers, it thought it was the same network, and I am guessing that it stored some signal/signal level information from the test router, which the ASUS did not match. To test this, I created an additional 5g guest network with a different SSID, and it connected fine. So, you may ask, why did it work on the regular 5g connection? Because I never used the regular SSID on the router I was testing. Basically:
The stick was used on the regular SSID only on the ASUS router, never on the test router, so it worked when I switched back from the test router to the ASUS.
The stick was used on the test router, but only on the guest SSID. It worked on the test router.
I then created my first 5g guest network on the ASUS, using the same SSID as was on the test router. When I tried to switch the stick to this guest SSID on the ASUS router, it failed to connect.
I create a never-before used SSID for a guest network on the ASUS, and the stick connected.
* A new version of the Fire Stick firmware was delivered today, and it now has an option to forget networks. I do not plan on trying it, as what I have configured now works fine.
