I had the same issue and contacted T-Mobile customer care. If you only have home internet from T-Mobile, then you cannot get access to My T-Mobile account online. You will have to call the T-Mobile Customer Care to change autopay credit card or change billing info etc. if you already have a T-Mobile cellular line, then you will be able to log into My T-Mobile account and it will also show your T-Mobile home internet line.
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