iPhone 6 Won't Switch to 3g when 4g LTE Signal is Poor

I have AT&T and generally get pretty good LTE service around my area. However in my office which is on the 6th floor of a building in downtown Hartford, I consistantly only get 2 bars and little to no data throughput. If I manally turn off LTE then the phone will switch to 4G and I get 4 - 5 bars with excellent data speeds. I have a window seat and I notice that if I move towards the center of the tower then the phone will switch on its own. However at my desk it will stay on LTE even though there is almost no data throughput. Why won't the phone recognize that it has a very slow data connection and switch to 3G? Is there a problem with the phone or is there a setting I can change?