You can set any of your SIM cards to automatically refill / top-up when they have consumed all data. This means that your SIM card will always be online. When all data is consumed we'll charge the card you've entered.
You can also manually refill and change to a different plan. Learn how to do that here.
Video walkthrough
Steps
Login to the Wave Connect portal
Click SIMs on the left navigation
On the SIM you want to set to auto-refill, click the Auto-refill switch and a modal window will appear
On the modal window, enter your payment information
Click the Save button