Auto-pay and Bill Cycles are preset to certain periods of the month for TMobile accounting purposes. You shouldn't have a full month charge for a partial months use that is why sometimes you see a month-and-a-half and sometimes only half a month on the bill when you first start Carrier Services.
Hey there, and welcome! I'm so sorry to read that we didn't do a better job of onboarding you regarding our billing when you started your service -- that's a definite opportunity, thanks for taking the time to post here and let us know!
Billing with T-Mobile is called Bill Current -- you're billed at the beginning of your bill cycle, and due dates for recurring charges are within that same cycle -- a little ahead of your cycle close date. For example, if your bill cycle runs from 1/1 - 1/31, your due date might actually be on 1/21. "Usage charges" (charges for things like international calls or play store purchase that are billed to your account, for example) aren't billed until the following month.
I hope that explanation helps a bit, but please let me know if you have any more questions!