RBR50v2/RBS50v2 RBR accesses 5g, but RBS is running 2.4. How can I get them to both run 5g?

"accesses" is a bit ambiguous.  Can you please describe the situation in more detail?

Both the Orbi router (RBR) and satellite( RBS) are supposed to broadcast the Orbi WiFi name (SSID) on both a 2.4G WiFi channel (1-11 in the US) and a 5G WiFi channel (36-48 in the US).  Most modern WiFi clients display only the one SSID and do not indicate how many access points are broadcasting that SSID or on which channels*.  After a device connects, many devices have a way to display the properties of the connection (channel, link rate, etc.)

What is the basis for concluding that the satellite is not broadcasting the 5G WiFi?

* There are software applications that will display the characteristics of every WiFi access point (signal level, channel, etc.)

For Android, I am partial to WiFi Analyzer by Kevin Yuan.

For Windows, WiFiInfoView by Nirsoft.

(both are free)