I'm trying to connect an old RBS50Y in extender mode to the RBRE960

Thank you all. I finally got it working. I had to switch it a couple of times between Orbi Satellite mode and Extender mode before it actually worked and went through the whole process. Also you need to be patient, it takes a LOT of time, even more than with other Orbi units ;-). 
Didn't have to load any FW, extender mode was already built into the FW I have. 

The critical point is that you cannot configure it fully manually, as I like to do. You HAVE to let this "Genie" thing do its thing to select and connect to a host SSID. If you choose manual setup, you can configure the broadcasted SSIDs but there is nowhere an option to say which host SSID you want to extend. At least nowhere I searched ;-).

The whole setup is imho rather clumsy and buggy, but OK, I finally got managed to set it up the way I wanted: just une single 2.4 SSID broadcasting.