It was said here, RBR850 and RBS350 did not work together

The Netgear Knowledge Base article is clearly correct, but "details matter".

• As the footnote indicates, the 350 product was created to be a lower cost alternative because it is a dual band product, rather than a tri band product like the 850.  Whereas the 850 uses the lower 80MHz frequency band for user devices (5.18GHz to 5.24GHz) for user devices and the upper 80 MHz frequency band (5.745GHz to 5.805GHz) for router to satellite communication, the 350 product uses the upper frequency band for both user devices and router to satellite communication.  That means that every data packet to or from a user device on a 350 satellite takes up radio time for both the communication between satellite and user and between router and satellite.  The radio time taken for satellite to user device will also impact the router to satellite connection between the 850 router and any 850 satellites.
  
  So, yes.  They are compatible, but cannot produce the same maximum performance as an all tri band solution.
  
  5G WiFi has a lot of capacity and in practical terms users might not ever notice.  It takes only 25MHz to stream 4K video.  Maybe an all 850 network could have a dozen televisions streaming 4K video and this hybrid could support only a half-dozen televisions.  Who cares?
• A different concern is being careful about firmware releases.  Netgear once sold two packages (RBK652S and RBK653) which consisted of one RBR750 and one or two RBS350 satellites (less costly than two RBS750 satellites).   Users have posted on the forum about issues when they installed the latest firmware on the RBR750 and found that it caused problems.  The solution was to keep the RBR750 at the firmware release listed in the support page for those packages. Since Netgear never released a package of one RBR850 and RBS350 satellites, I have no idea what happens when later firmware is put on the RBR850 router.  Will the RBS350's still work correctly?  No idea!

Another consideration is those fortunate customers who can connect satellites to the router with Ethernet cables. When the router to satellite communication takes place over Ethernet, the performance issue largely disappears.  But, what about the firmware compatibility issue?  (again, no idea)