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.
When first booting up the Atmoph Window 2, the plan selection page will be shown as the image below, so choose the plan you'd like to join. To check which Limitless plan you've currently joined, go to MENU > Account > Payment.
To start the Limitless plan, you need to register a credit card.