Orbi 960 not obtaing a DHCP address from upstream carrier 5G modem/router.

Does the ISP router show the RBR as being connected to the ISP router on it's web page somewhere?

Does the RBR show a connection rate being seen on it's web page under Advanced tab, show stats button? 

Wondering if connection rate between RBR and your ISP router isn't working right. 

Happen to have a non managed 1Gb LAN switch that you could put in between the RBR and your ISP router and connect only these two items to this switch? 

Your ISP Modem already has a built in router and wifi. This would be a double NAT (two router) condition which isn't recommended. This would be a double NAT condition which isn't recommended. https://kb.netgear.com/30186/What-is-Double-NAT

https://kb.netgear.com/30187/How-to-fix-issues-with-Double-NAT

Couple of options,

1. Configure the modem for transparent bridge or modem only mode. Then use the NG router in router mode. You'll need to contact the ISP for help and information in regards to the modem being bridged correctly.

2. If you can't bridge the modem, disable ALL wifi radios on the modem, configure the modems DMZ/ExposedHost or IP Pass-Through for the IP address the NG router gets from the modem. https://kb.netgear.com/25891/DMZ-on-NETGEAR-routers

https://kb.netgear.com/24086/How-do-I-set-up-a-default-DMZ-server-on-my-Nighthawk-router

3. Or disable all wifi radios on the modem and connect the NG router to the modem, LAN to LAN configure AP mode on the NG router.

https://kb.netgear.com/20927/How-do-I-change-my-NETGEAR-router-to-AP-mode

Procedure to configure Static IP Address for AP Mode for NETGEAR Orbi or Nighthawk router systems. | NETGEAR Communities

Try option #2 first...

If normal connection doesn't work, try option 3 for AP mode. 

Your also behind on FW as well...something to update FW and see...