Ah I see. Yes if you have the mobile version you need the yearly care package to sync it with other devices via their cloud service. I'm not sure if any of the other options I listed offer similar multi device packages.
I get round it by using a small laptop on site then plug it into a dock at home to use with a standard keyboard and monitor, so only one license and no mobile version. Only bought one years care package in four years, and that gave me the 18th edition updates. So total cost for four years has been £280-ish inc vat.