It could be a compatiblity issue but it could have to do with the encryption too.  I would try as a test to open the network temporarily and see if it is seen then.  If not we would need the model, version and FW version of both the RE and the AP to see if we can figure out if a hardware or software issue exist. 

So I took the 5G AP radio to 20 MHz and it worked. Problem is, the wireless AP 5G speed drops by a factor of 600 MBs to around 120 or less. 

 Not really worth the effort on cox Gb internet plan and using this extender to extend network to a remote office. 

 Going to have to use Ubiquiti products going forward. TPLink stuff is very limited in the top plan equipment it seems. 

 I would Change the specs to mention this extender is very limited in it 5G range.

@Maruefer 

If you had to change the channel width to 20 MHz that would indcate interference or range.  Also narrowing your bandwidth will drop the speed.  Also Range Extenders regardless of manufacture will cut network speeeds around half of what you'd get dirrectly from the AP.  This is becasue of how a range extender communicates with the AP and client devices.  The Range extender can only communicate in one dirrection at a time, thus the reason for the reduced speeds.  In a situation like your I would recommend going Mesh.  You will have much more success with that.  Try any of our Deco products that have gigabit ethernet.  

Best Regards