RBS750 favors "poor" 5G over "good" wired connection

There is no setting for satellite to router backhaul link. It is automatic.  When the router detects an ethernet link to a satellite, the connection is changed to 'wired'.  If the wired link disappears, the backhaul connection is changed to 'WiFi'.  (and, if the 5G WiFi link goes bad, then the backhaul link is swiched to the 2.4G channel.

I sense two possibilities:

1. Something is wrong with the ethernet cable or wall jacks.  The test for this is to temporarily move the satellite into the same room as the router and connect them with a good ethernet patch cable.
2. You have been caught by the "version 4 firmware" problem.  Although the firmware you mentioned is the most recent on the Download page, there is a newer version which is still in the Quality Assurance testing process that has been provided to a number of customers.  Check this post:
3. https://community.netgear.com/t5/Orbi-WiFi-6-AX-and-WiFi-6E-AXE/New-RBR750-RBS750-Firmware-Version-v4-6-7-5-Released/m-p/2201654#M29340 
   Somewhere toward the end of those 202 messages is a link to this newer version.  You might want to try installing that version and see if it helps.
   At the very least, reading through the messages will provide some perspective on the 750router/satellite backhaul link.