MR60 slows down and drops connections frequently

I have an ATT FIber modem/router (BGW210-700 ) with 1000mbps download  speed that I have had no issues with whatsoever. I recently got an MR60 for more control of my network and having some issues.

On my BGW I Disabled Wifi, enabled IP Passthrough, and set IP to DHCP-Dynamic. initially everything worked good for a few day. I had good range, i was getting 900Mbps download speed in the same room, 200+ at the further point from the Mr60 router, then I noticed that I had really poor speeds, sites weren't loading. When doing a speed test I was getting 2mbps download speed. Tried connecting to the nighthawk app and it was constantly timing out. Power Cycled the mR60 and i was back to normal. Hours later, the same thing started to happen again and i noticed devices i was on the wifi would drop out or say no internet and then reconnect.

At this point, searching around, i thought maybe it was a double nat issue and i set my MR60 router to AP mode and powered off and on again. Seemed to be okay, but then i noticed connections dropping again, slow speeds, and couldn't connect to the nighthawk app again.

At this point, i disabled SSID broadcasting from my guest network and power cycled again.

I'm not sure whats going on, this is my second nighthawk router and the first one was returned for similar issues.

My BGW210-700  appears to be fine, when i revert my settings and don't use the nighthawk, i don't have these issues.

Can anyone point me into a direction to stop this from happening?