DavidTN wrote

@TP-Link  I've attached a photo of the screen showing the error message. I already had Fast Roaming disabled, and it didn't make a difference. I tried disabling Beamforming, and at first it didn't appear to work because I got the same error message. But when I dismissed the error message, it asked me for the WiFi password (which it wasn't doing before), and then it connected. For the security setting, I had to select WPA/WPA2-PSK. Anything else made it disconnect. 
 

I'm currently streaming a movie from Netflix to see if it will stay connected, but so far it seems OK. 

@TP-Link, I was successfully able to stream two complete movies from Netflix with no problem. I think I have arrived at an acceptable solution.

Just to summarize, for anyone else who's reading, my two Apple TV (Gen 3) units could not connect to my new Deco X20 system out of the box. I tried various secuirty protocol settings as well as alternating between router and access point setups. Nothing worked. Based on TP-Link's suggestion, I disable Beamforming in the Deco settings (Fast Roaming was already disabled). After doing this, I still got an error message when selecting the network in the Apple TV, but after dismissing the error message, it asked me for my WiFi password and then connected. I don't know if router/access point settings make a difference, but I am currently running in access point mode because my router existing router has better parental controls.

Thanks for the help, @TP-Link!