I have a similar issue with my RBR850 but I'm on V7.2.6.21

How are the RBS ethernet connected? Directly behind the RBR or is there a LAN switch here? 

What CAT# LAN cable is being used between the RBR and RBS? 

FW is here:

Orbi Firmware Releases | NETGEAR Communities

I recommend manually updating, RBS first, then RBR lastly using this process:

Procedure to Manually Firmware Update Orbi and Nighthawk Mesh systems. | NETGEAR Communities

I would probabably do a full on factory reset on the system using this process:

Orbi Systems Manual Factory Reset Process | NETGEAR Communities

I would work with the RBS first in wireless BH mode, can bring the RBS to same room or near(er) to the RBR. Then once system is fully set up again, and wireless connected, ethernet connect each RBS to back of RBR to make sure they get synce via ethernet, then you could put them back in there remote locations. 

I had my 853 series online for over 30 days, saw no ETH BH issues with my RBS. Non managed LAN switches in the mixe as well. 

Share your Orbi 350, 750, 850 or 960 series system UpTime | NETGEAR Communities