Try using the KMS key here as intermediary step
https://learn.microsoft.com/en-us/windows-server/get-started/kmsclientkeys
Dism /online /Set-Edition:ServerStandard /AcceptEula /ProductKey:xxxxx-xxxxx-xxxxx-xxxxx-xxxxx
then after reboot
slmgr /ipk xxxxx-xxxxx-xxxxx-xxxxx-xxxxx
slmgr /ato
--please don't forget to Accept as answer if the reply is helpful--