When you made the Tracfone SIM the primary data slot, you moved it off GSM (2G) for data and onto the fastest available, hence the LTE. No, Tracfone is not using VoLTE to the best of my knowledge...
On dual SIM Windows Phones, you choose one slot for data and that gets use of the 3G and LTE radio, the other slot can have voice and 2G data (useful for SMS/texting).
Don't know where your FreedomPop data went. Have you tried contacting them?
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