By default, RDP access to a server or client is always disabled and needs to be turned on manually. In a domain, you can use a group policy to enable it via GPO but there are several settings to be configured before it works. If one is missed, you may end up with the Enable Remote Desktop slider set to off and grayed out:

Configure the following items in a group policy to properly enable Remote Desktop access:

• Computer Configuration → Policies → Administrative Templates → Windows Components → Remote Desktop Services → Remote Desktop Session Host → Connections
  Allow users to connect remotely by using Remote Desktop Services = Enabled
• Computer Configuration → Policies → Administrative Templates → Windows Components → Remote Desktop Services → Remote Desktop Session Host → Security
  Require user authentication for remote connections by using Network Level Authentication = Enabled
• Computer Configuration → Policies → Windows Settings → Security Settings → Windows Defender Firewall with Advanced Security → Windows Defender Firewall with Advanced Security → Inbound Rules
  Create a new rule, select the predefined Remote Desktop, then on the next page, enable all 3 ports.
  Optionally, after creating the rules, you can narrow the scope and/or profile (e.g. Domain Profile only) by double-clicking the newly created rule and adjusting the settings on the various tabs.

Reboot the server(s) after configuring the group policy, trigger or wait for the group policy update to complete.