Att still charges me even after i have cancelled my account and transferred to another carrier back in April!!! Calls were not answered

So I transferred my cell phone number to another carrier back in April. I called last month about the issue of still receiving a bill from AT&T, and I was told that my account had been closed and that I would not receive any more charges, however, I still got a $16 bill in my email today! I don't understand why it's $16.
The only thing I think might be the reason was that I went to the AT&T store in April and asked them to get my Apple watch cellular works.
The staff there charged me $10 and told me that my account is a prepaid account, I have to switch to a postpaid account so that I could use the cellular. and he said it is too complicated to switch account types in the system, so I'd better transfer my account to another carrier and then transfer it back. So I did it and I found out that another carrier could activate my Apple watch's cellular directly without any issue or extra effort! So I decided to stay with the other carrier.
Now here is the problem, the customer service won't pick up my calls and I was being told that my account is deactivated, and for the online services I could not log in since I no longer have access to my account. However, I'm still receiving emails from At&t and I'm so annoyed with the issue. Could someone just explain to me why r they still charging me even after I canceled back in April? and will they still charge me next month? why after I've canceled my account but still being charged for the apple watch cellular service???? Should I go directly to the store and ask them to stop charging me??? I can not turn off the auto-pay for the cellular service, I don't even know my account password! Thanks in advance!