What is the AT&T APN for the US

[ScruffR]

Or is this a special m2m SIM from ATT?

In that case you might need

Reasonable 3rd party data provider for Electron w/ 3g service?

Alright, just following up on this in case others come looking and find this thread: The AT&T sims are working great. You will need to set your APN to ‘m2m.com.attz’. OTA flashing works fine with the keepalive set to 30 seconds.

Or any of these might reveal the secret
https://community.particle.io/search?q=att%20apn

But this suggests my previous inclination towards Broadband wasn't too far off

Connect electron to internet without cellular coverage or via ATT Microcell

An AT&T nano SIM card (from an iPhone 5 or 6, for example) will fit in the Electron and should work, at least without the Microcell. To fully connect to the cloud you’ll need to insert this line in your code: STARTUP(cellular_credentials_set("Broadband", "", "", NULL)); Unlike Wi-Fi settings, the cellular APN is not stored in configuration flash so you always need to add that line to any program that uses a 3rd party SIM card. The string “Broadband” is specific to AT&T. Just take the line out …