Whom to contact to correct AT&T installation mistake

I signed up for AT&T U-verse 1000Mbps internet (with TV and phone package). The installation technician did not use CAT5e or better from ONT to the gateway. I get only 250Mbps because of the CAT3 voice phone wire used. I emailed the installation supervisor, but he refused to take any action. Calling AT&T service did not do any good. Whom can I contact to get AT&T to do the installation properly, or at least talk to someone who understands the cable rating issue?

My Windows 10 PC is sitting next to the gateway and hardwired by CAT6 to it. The ONT to gateway CAT3 cable is the bottle neck.