he tech mainly stood there while I did the install myself.
Yeah right... What else in this story? He took off clothes and stood naked while he watched you install.
Its correct that self-install has max available speed 12. Faster speeds require running new line from the box to your modem so they require professional installation. If I were you, I'd call att while he was still there and verified he got orders to install line for 18mb package. And there is always a charge with pro installation. You should have gotten at minimum also TV since you had pro install which you could cancel by the end of first 30 days and return box so this would have had gotten you installation charges waived. You'd only pay for activation fee and first month of tv package.