Bridge mode will lose internet to the main router every 20-50 minutes and requires a restart.

So, I wanted only one main router and to have the power to change a DNS server without paying for Xfinity XFi, I set the router to Bridge mode and it was connected to my Netgear R6250. It all seemed fine at first until about an hour later the internet goes out and it restarts itself in the 5 minutes of downtime and comes back. I thought this was just a rare occurence so I thought it was still fine. Except, this would keep going on repeatedly. I change it back to how it was before with Bridge mode off and it doesn't occur anymore. Neither seem to be overheating or any problems, what gives? How can I fix this?