With Checkfox, you receive a 10 EUR prepaid worldwide SIM card.
It is already inserted in the device.
To activate the SIM card and for the data costs, please go to thingsmobile.com, create an account and register your SIM for the pay-per-use plan.
The device does not use up a lot of data so the prepaid SIM card should last a while, but in case you need to recharge it, you can do it in this account too.
Or you can use your own data SIM card, of course, just make sure to change the APN settings accordingly.