While an active internet connection is required to be able to download a work area or work package, and also to upload your completed work, it is not required to actually perform your work.
If you are going to be working in an area where you either know or suspect that there may be issues with the mobile signal, you can download your work before you get to site in any location where you have a mobile or WIFI signal.
You can also use the personal hotspot function of your mobile phone to provide the tablet with an internet connection if your phone has a signal but the tablet does not.