LB1120 slow compared to iPhone

At my location I am line of site of the AT&T cell tower. From the 2nd fl I can see the cell tower panel antennas. The LB1120 has four bars lit.

I can connect an external yagi to the main or the main and diversity antenna ports yet the best download speeds obtainable is about 55mbps.

Yet, when I run the same speed test on my XR iphone I consistently get 80-90mbps. This seems odd. Please explain.