I had a "Pay As You Go" plan with T-Mobile with auto pay enabled ($30 each month). However, I took that phone number and ported it over to my AT&T SIM.
I now wanted to login my T-Mobile account and disable the auto pay, since I will not be using this SIM anymore, however I can't login anymore since there is no number associated with my account (obviously). Does this also mean that the Pay As You Go plan (including my auto pay subscription) has been terminated, or should I still deactivate things somewhere?
Unless you have a family plan, the account has been terminated.
Porting a number from One carrier to the other terminates the account on the first carrier. You can however call T-Mobile to verify that your account is no longer active and that pay as you go is canceled.
Manage your T-Mobile account