Wan Drops on router

Does the Internet LED on the router turn AMBER or RED when this happens? 

What Firmware version is currently loaded?
Be sure your using a good quality LAN cable between the modem and router. CAT6A STP is recommended.

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
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
Try option #2 first...

If the WAN port keeps resetting on all the routers you have tested, then this is a ISP issue with there modem/gateway. This means there is a loss of internet connection between the router and the ISP modem/gateway. Something they need to look into.