wire connect Orbi RBR50 in AP mode to ANOTHER RBR router with 2 satellites

I have an Orbi RBR 50 in AP mode wire connected to my FIOS router.  I also use two RBS 50 Satellites (the router is in the middle).  This is a long narrow apartment building with many rooms and extremely thick walls (built 1903).  One satellite is a bit far from the router and devices connected to it report not having a stable signal when connecting to Zoom and similar apps.  Router and Satellites have the most recent drivers.

I took the satellite that covers the other end of the apartment and put it between the router and the weak-signal satellite  thus creating a shorter distance between the router and the two satelites.  My signal appeared more stable with this arrangment.

To make this workable could I purchase a third satellite to cover the end of the house  which is without any Satellite?  Which model? 

OR - could I purchase and wire a new second ORBI RBR 50 to the existing RBR 50 so as to extend the signal?  Since the original RBR 50 is used in AP mode, I'm assuming the new second RBR 50 would also be placed in AP mode.

All suggestions and thoughts appreciated.