All license models now use HTTPS (port 443) to connect to the cloud licenses servers.
If you are in a highly restricted environment, your IT may need to unblock the following -
Allow ArcGIS Pro to connect to the internet on port 443 -
C:\Program Files\ArcGIS\Pro\bin\ArcGISPro.exe
And'whitelist' the URLs -
http://hostedactivation.com/
https://ls53.rlmcloud.com/
If you still are unable to activate a license, you can also try the offline activation method. Please see related article.
