We use Freemius 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