We can activate any modern Windows including Windows Server 2019 via proxy using:

Through server manager -> Local server:
1. Disable IE enchanced mode configuration
2. Configure correct timezone as per requirement
Configure proxy in Internet explorer such that you can open https://www.google.co.in. If proxy requires authentication enter username and password and also save them.
Then Try to activate windows. If you leave system running for 15-20min without closing IE where we saved authentication and proxy configuration, Windows should get activated.

Offline activation via phone

Other option is to run either:

  slui 4

OR

  slmgr /dlv

and use the activation ID generated 9 sets of 7 digits via telephonic call to Microsoft for activation.

Anonymous