Does that mean you can't use WhatsApp if you don't have WiFi or does it use the data associated with your existing number?
You could configure the WhatsApp account to show as the devyce number, but the connection would be made through your existing numbers connection or WiFi depending on how you were connected.
If you have WhatsApp installed on the existing phone and active on that number then the WhatsApp messages would show as the ‘genuine’ number unless you are able to successfully reconfigure it to the other number.
It could be risky to get right without exposing your real number.
I use TextMe for virtual numbers, and though I’m not in a relationship to protect, I still wouldn’t want to be mixing the use of a virtual number with different apps.
The Devyce website doesn’t show how it manages the WhatsApp connectivity. If it just provides the number then you are using your phones WhatsApp and it will use the number configured for the account - and you can have only one number in it at a time
If the Devyce app itself contains a separate internal WhatsApp then you could have your standard WhatsApp app with your normal number and your Devyce number running through your data connection