This was eventually resolved after 2 (useless) store visits and 6 (mostly useless) calls to T-Mobile support. They had to unlink the secondary number from the new T-Mobile ID, then I had to log into the original T-Mobile ID and enter the phone number for the secondary line under my original T-Mobile ID. After this was complete, I then had to change all the account info (name, email, etc) to the information of the secondary line holder. If this had worked the first time before I had to create the new T-Mobile ID, I would not have experienced this issue. According to T-Mobile support, this is a common issue when the primary line gets ported out/cancelled on a multi-line prepaid plan.
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