We use Freemius as a reseller to handle our licenses.

Please refer to this article about the common issues for license actions:

https://freemius.com/help/documentation/wordpress-sdk/license-activation-issues

If you need to whitelist IP addresses, you can find the necessary IP addresses from here:

• api.freemius.com:
  • https://mxtoolbox.com/SuperTool.aspx?action=a:api-1122186795.us-west-2.elb.amazonaws.com&newAppVersion=1
• wp.freemius.com:
  • https://mxtoolbox.com/SuperTool.aspx?action=a:wp.freemius.com&newAppVersion=1