RBR50 as extender to RBRE960 + (2) RBSE960

FURRYe38 is entirely correct that the original Orbi "50" router and satellite are not compatible with any of the AX product line.  If there is part of your property that is totally outside the coverage area (such as a barn, boat dock, etc.) then the old system could be redeployed to that location to create a separate WiFi network at that location.  The major obstacle to that use is that the Orbi router WAN port must be connected with Ethernet, either a physical cable or a wireless bridge that appears to the router as Ethernet.  Users who need remote coverage most likely have already addressed the issue, so my guess is that you're stuck.

Now that WiFi has become essential to our daily lives, it might be worth taking a minute to consider "what's the plan?" if the new WiFi router fails.  If you can drive to Best Buy and have a replacement running in a couple of hours, that's a plan.  Otherwise, it might be worth keeping the old Orbi setup in a box "just in case."