I have determined that one cannot link a prepaid and postpaid T-Mobile account to have one login ID. I clicked a button on the T-Mobile ID support page that said it allowed linking a number to an existing ID. The button didn’t work at all when used while logged into my existing prepaid account, doing weird things like redirecting me through various URLs and ending up on the prepaid home page or on an account page that would never load the main content area. So instead, I created a separate ID for my postpaid account, then linked the prepaid number to that. And it worked, in that it connected the one to the other.
Except that it didn’t work, in that I could no longer get into the prepaid account at all. I couldn’t log in with my old credentials. When logging in with the new credentials, I was presented with the ability to switch accounts, but clicking on the prepaid one would either take me to a 404 not found page, redirect around and end up on the prepaid home page, or ask me to enter my pin, then freeze when I entered it. I tried many times both on the website and the T-Mobile phone app (which has never worked for my prepaid account).
I had tried a web chat with T Mobile support before I found and tried that link, to find out if joining accounts could be done. The person had said that they didn’t have access to the prepaid system and I would need to call in to the prepaid support. After finding that support page and getting myself stuck with an inaccessible account, I looked for a prepaid version of the chat, and couldn’t find one, so I called. After talking to the person there and getting her to understand the situation, and her discussing with her support people, she informed me that the two types of accounts could not be joined, and detached them for me. Attaching the prepaid to the postpaid had basically deleted the prepaid login ID, so I had to create a new one. For some reason, the phone conversation got cut off with support, but I was able to create the ID just fine and get back to having two separate accounts.
A little more management with the accounts being separate, but it shouldn’t be a problem. It’s sort of weird how separate the systems are, but so it goes, I guess.