hi @Christos1080 , happy to have you on board!
To answer your questions:
3. Yes, you can use iotcreators sim cards under cosmote in greece (nb-iot only, lte-m not supported)
4. yes absolutely. Also, if you want to use iotcreators services, the only way to access them is to use iotcreators sim cards
5. I searched for your modem and it seems that supports nb-iot and lte-m, so itcreators sim cards should work

If you have any other questions, do not hesitate to ask! 😉

Best Regards