Thank you very much for the reply, Georgie!
Yes, I should've clarified that I was also in Germany briefly (Munich Flughafen) but unfortunately could not use the data.
I'll note for next time to use a Swiss-Sim and completely forgot that Suisse isn't part of EU! Thanks for that clarification.
I suspect when I bought the SIM from T-Mobile and they took my US Passport, they possibly registered me as non-resident, even though I have a job here and will be leaving here for the next year or more. Do you think I could change my residency status with T-Mobil to allow me to use the data in other EU countries? Thanks!