If I have a Flexy here in US equipped with the 4G-EU extension card and an AT&T SIMM, can I test this fully here?
That’s going to depend on what bands the AT&T tower near you is using. It is not recommend.
this work in a Cozy, are the modems different internally for Flexy205 ?
The “EU” labelled modems are designed for European carriers and the spectrum used in that part of the world. It’s use is not recommend and potentially violates FCC rules in North America. That said, some of the bands are the same in both areas, so it may or may not work depending on a number of factors. Click link below to learn more:
Can you explain why it works in a Cozy?
Some bands are common between US and EU cell providers, so you may have a tower nearby using one of those bands.