I thought the iPhone SE (2016) was GSM compatible?

[GGD]

What's the model number of this SE (Settings>General>About)?

Here are the different models and the bands they support, what carrier are you trying to use? Any chance that the phone is "carrier locked"?

https://www.whistleout.com/CellPhones/Gu...er-carrier

There certainly are 2016 SE models that work on GSM, I have several on AT&T, they were purchased as "Unlocked".