Airport Extreme does not give option to join network
I am trying to set up a repurposed airport extreme in bridge mode. Previously I have always been able to use the "join network" option but now it is only giving me the option to create, extend, or off. I have a model a1521 (6th gen) that I have as my main router. I also have an older airport express that I have in bridge mode. I am trying to add the a1143 (3rd Gen) to the network and just put it in bridge mode. Purpose is to use it as a wireless print server until I can move my printer and 6th gen to the same location. Was this option removed? am I just missing something?
Mac mini, OS X El Capitan (10.11.4)