Horrible customer service experience

File a BBB complaint and someone higher up at AT&T will contact you or you can log on to your online account and change the plan.

What do you mean by “the wrong plan” How exactly was it wrong?

When you add a service or change your plan, read the next bill in full to make sure everything went as expected so you don't suddenly find out months later.

You're past the normal timeframe for dispute as any company must be notified of a billing concern within a reasonable time. You could try a BBB complaint, but be prepared they might just stick with "well you waited 4 months so you've accepted all charges".