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. 

@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