Needing help finding satellite add-on for Orbi RBR750

We have a 4600 sq ft home and we are using the Orbi RBR750.  For some odd reason, we have begun having issues with wifi on one end of our home.  This end of the house consists of the guest bedroom and the garage. The wall dividing these two areas is where one of the current satellites is sitting.  Everything has been fine for the past year, nothing has changed other than the performance is slow, sometimes not connecting just recently.   It seems odd that I should have to purchase another satellite for this area (keep existing in the bedroom and put the new one in the garage) but if it fixes the issues I'm all for it.  

Does anyhow have any advice on this or can you offer which satellite I would need to purchase that would work with this system? I appreciate any help and input. Thank you.