Well, it is still possible to do an RDP. Somehow, as you concerned, the RD Connection broker does not work properly in a workgroup environment. A configuration that does not use the RD Connection Broker role service provides desktop sessions to users based on the number of Remote Desktop Services client access licenses (RDS CALs) that are installed on the server. Such a configuration does not provide access to RemoteApp programs or the RDWeb website. Because a configuration without the RD Connection Broker role service does not provide access to all RDS functionality.
I'd like to recommend that you install RDS in a workgroup through the link below.
This article provides guidelines to install and configure the Remote Desktop Session Host role service on a computer that is running Windows Server 2019 without the Remote Desktop Connection Broker role service installed.