Is the project area in the same radio configuration zone (generally; on the same continent)?
If the project area is on a different continent, that continent might use different frequencies.
If the project area is in the same radio configuration zone, the issue is probably that the coverage is insufficient.
Are you using the device indoors or outdoors? Use the coverage map at Sigfox Backend to get more detailed information on the coverage, and use the simulation for indoors if you are indoors.
If you can post the address of the project area here, we can help checking the coverage.