Connecting RBR750 router cables to New Modem C4000BZ?

Your ISP Modem already has a built in router and wifi. This would be a double NAT (two router) 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 Orbi 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 Orbi router gets from the modem. Then you can use the Orbi router in Router mode.
3. Or disable all wifi radios on the modem and connect the Orbi router to the modem, configure AP mode on the Orbi router.
New: https://community.netgear.com/kb/en-orbi-knowledge-sharing/static-ip-address-configuration-for-ap-mode/2456256
https://community.netgear.com/t5/Orbi-WIFI-6-AX-AND-Wi-Fi-6E-AXE/Procedure-for-Factory-reset-of-RBS850/m-p/2367505/highlight/true#M53985
https://kb.netgear.com/31218/How-do-I-configure-my-Orbi-router-to-act-as-an-access-point and https://www.youtube.com/watch?v=H7LOcJ8GdDo&app=desktop 
Try option #2 first...

Any connection on the RBR with ISP services needs to use the Yellow WAN port in back of the RBR connected to the ISP service modem.

Review this as well:

Orbi MESH Troubleshooting, Suggestions and Tips | NETGEAR Communities

Wow, I was amazed by your response, thank you. I've got some homework to do, :-).  Again, thank you very, very much!