Unable to extend Orbi RBR50 wifi with EX7700 – AC2200 Nighthawk X6 Tri-Band WiFi Mesh Extender

Hi I have an Orbi RBR50 router (firmware v2.3.5.30) and two Orbi RBS50 mesh satellites which all work happily. I am now trying to add an EX7700 nighthawk (firmware v 1.0.0.202 - latest) to the mesh, ...