If there was a way to work offline, you'd be more likely to run into conflicts where other people may have changed the same profile, or perhaps merges needed, if you add a profile that somebody else adds while you're offline.
You can export a GEDCOM file, I haven't tried it and don't know how it works. I assume you select the desired profiles somehow. Then you could work on that part of the tree with the genealogy program of your choice. Bringing the data back to WikiTree would be a bit tedious however, and have the usual problem with importing sources from a GEDCOM.
I do most of my work at home on the laptop. With home internet it's a flat monthly charge. Data charges only apply for use with phones (at least in the U.S.) and I would have a hard time working on WikiTree on a phone. I need to have multiple tabs, big screens, etc.