You can join the Limitless plan directly from the Atmoph Window 2. Be noted that you could not subscribe or cancel your plan through smartphones or PCs.
To start the Limitless plan, you need to register a credit card. Then, go to MENU > Account > Payment > Limitless and choose the plan you'd like to join from the plan selection page as the image below. When using Atmoph Window 2 for the first time, the plan selection page will be shown during initial setup and choose the plan you'd like to join.