Here is the easiest way for you to activate the License Key for the plugins!
Divi Next Website → Your Account
Sign in to your account at this URL: https://www.divinext.com/your-account/ and then, copy the License Key from your account.
WordPress Dashboard → Plugins
Head to your WordPress Dashboard and go to Plugins, here you paste the License Key onto the License Key field
Save Changes → Activate License
After applying the License Key to the License Key field, you have to first click ‘Save Changes’. And then click on ‘Activate License’.
Here’s a video that shows you the process as well: