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

What is the Mfr and model# of the Internet Service Providers modem/ONT the NG router is connected too?

What is the size of your home? Sq Ft?
What is the distance between the router and satellite(s)? 30 feet or more is recommended in between RBR and RBS to begin with depending upon building materials when wirelessly connected.
https://kb.netgear.com/31029/Where-should-I-place-my-Orbi-satellite

What channels are you using? Auto? Try setting manual channel 1, 6 or 11 on 2.4Ghz and 40 to 48 channel on 5Ghz.
Any Wifi Neighbors near by? If so, how many?

Try enabling Beamforming and MIMO(MIMO may or may not be needed) and WMM. Under Advanced Tab/Advanced Settings/Wireless Settings

Try disabling the following and see:
Armor, Circle, Daisy Chain, Fast Roaming, IPv6 and Set 20/40Mhz Coexistence to 40Mhz only. Set Short preamble instead of Long preamble modes. Save settings and reboot the router and satellite(s).

One User Experience/Configuration:
https://community.netgear.com/t5/Orbi/Most-Stable-Orbi-Configuration/m-p/1941087/highlight/true#M97026

Has a factory reset and setup from scratch been performed since last FW update?