My Xfinity Modem/Router is acting up with "Bridge Mode."

Xfinity Gateway: CGM4140COM

Hello, I've lately been having issues with my new Netgear Nighthawk Pro Xr700 and my old Xfinity Gateway. I did this upgrade since the Wi-Fi had a longer range on the new one and it was just better in almost every way. Ever since I had gotten the new router, the connection hasn't been that great and the download speed was much slower than it should've been. I did some research to dig up what's causing the problem. I think I may have found out. There's a thing called "Bridge Mode" on the Xfinity thing where you log-in as admin. I've been trying to enable this feature over and over yet it always just goes back to being disabled. If this is a bit unclear then let me explain in a simpler way. I hit enable and it waited the whole 90 seconds and then instantly went back to being disabled as if it didn't work. I was wondering if anyone has the solution to this. I would really appreciate it and it would be a huge help. (P.S. If needed, I can hand over more info on anything required.)