2.4 GHz Band Stops Working

What model router is this? RS###?

CAT6A STP is recommended between modem and router.

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...