Mr60 having issues working with AT&T BGW320-500

What Firmware version is currently loaded?
Be sure your using a good quality LAN cable between the modem and router. CAT6 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

Try option #2 first...

 

Has a power off for 1 minute then back ON with the ISP modem and router been performed since last update?
Be sure to restart your network in this sequence:
Turn off and unplug modem.
Turn off router and computers.
Plug in and turn on modem. Wait 2 minutes for it to connect.
Turn on the router and wait 2 minutes for it to connect.
Turn on computers and rest of network.