Ok, thank you! I'm gonna gather as much documentation as I can.

Considering the fact that I get full speeds everywhere whenever I'm using a VPN, am I right to assume that there is an issue with AT&T's internal routing? And, that issue doesn't effect every path? I'm not really an expert at doing networking stuff, but I wanna gather as much empirical data to construct a report and do statistical tests n stuff.

My first guess wouldn't be routing, but traffic-shaping.

Perhaps the VPN you use is on a protocol/port that isn't outright rate-limited and since ATT can't peak inside your tunnel to see what you are doing with the bandwidth, it avoids any QoS/shaping/limiting that your non-VPN connection is subjected to.