Mr-1100 nighthawk with google mesh

Hi I have a mr-1100 nighthawk with att SIM card, with unlimited data and no throttling and I was getting speeds about 50-70mbps, but it would drop speed constantly, so it was Recommended that I purchase the google mesh router setup, so I did, when I first hooked it I got and incredible speed of 98 mbps, but after I configured everything the the speed does not get higher that 14 mbps. I set the nighthawk with a video that the people where I got it all from. I’m now lost and looking if anyone has this setup and how it works and if anyone can help me get higher speeds.