I've got Fast Roaming, Beamforming, and OFDMA/MU-MIMO all Enabled.

Another thing I didn't mention, was that I have the Deco system in access point mode.

... and the 6 GHz and 2.4/5GHz all have the same name, and I'm using a wired backhaul for each node.

I disabled both beamforming and fast roaming, and now the onn. 4K Pro can connect and stay connected for 30 minutes.

Edit: after an hour I noticed the WIFI had disconnected again.  I connected ethernet for a while, and that was fine, and now I'm back to WIFI to make sure it'll fail a second time.  This time I'm watching live TV so I can notice when it fails.

Edit #2:  I watched TV for a while, and the WIFI stayed connected.  When I paused it and it stopped streaming, the WIFI would connect/disconnect repeatedly, and eventually the Google TV would stop trying to connect.  

With developer options enabled I can see NETWORK_SELECTION_TEMPORARILY_DISABLED, 53m 23s, NETWORK_SELECTION_DISABLED_CONSECUTIVE_FAILURES=1

 Does TP-Link have any other ideas that I should try?

ChrisF4 wrote

 Does TP-Link have any other ideas that I should try?

  @ChrisF4 

I am not from TP-Link, but I have an idea. I am not familiar with this set top box, but according to review I found on the Internet that device has Ethernet port:

If set top box is so close (5 feet away) from Deco unit, just connect it to Deco with Ethernet cable.