Mobile Download speed slow in area

This issues had be going on for the better part of 3 months. As the title suggests, I have had VERY slow download speeds on my iPhone once I’m out of WiFi range. No it is not my phone, yes I have switched out the SIM card. I drive for Uber, and had a passenger on the phone and when we passed by area, they were mentioning that they couldn’t hear other party on call. I asked them if they had att, and they said yes. This is not a large area but it is very bothersome, especially when it interferes with my work. Image showing dropped speeds that image shows both coordinates of issue and the speeds I’m getting from att. I have attempted to “mark the spot” but I am sure that’s not doing anything (and application seems very outdated).  Please help get Att attention and fix this problem please.