Hi,
Are you able to see your license server's name (Active License Manager) under the table of ArcGIS Pro Concurrent User License in Licensing Page? If you are able to see it, it means you connected to your license server successfully, otherwise, you hadn't connected to it yet.
And also, I noticed on ESRI website https://desktop.arcgis.com/en/license-manager/latest/supported-software-products.htm#GUID-050952EB-C... that License Manager 2021.0 can support software products "ArcGIS Pro 2.8, 2.7.x, 2.6.x, 2.5.x, 2.4.x 2.3.x, 2.2.x, 2.1.x, 2.0.x, 1.4.x", which is not "2.8.x" but only 2.8.
In addition, only for test purpose, you can try to install it on your laptop. If the issue is gone, it means that your firewall setting still has some issues.