Essentially yes. If you don’t have access to mobile data on your phone and the venue doesn’t provide free WIFI you’ll have to generate your membership ID at home before you leave for the event. Don't forget to keep the browser window open so the page doesn't automatically refresh when you reopen your browser. Membership IDs are available to generate from 8 AM on the day of the event.
You can also log in to your Subba-account on a friend's smartphone to generate a membership ID.