Fix RDS connection from external PC

Current config/environment:
- A Nginx reverse proxy has a public IP and is forwarding remotedesk.abc.de port 443 to rdwa1.abc.local and rdgw1.abc.de to rdgw1.abc.local in the LAN.
- The reverseproxy is also streaming port 3389 to the rdgw1.abc.local.
- remotedesk.abc.de is resolved to the sam public IP as the reverse proxy.
- rdgw1.abc.local is a Windows 2019 Server with the Remotedesktop-Gateway installed.
- In the LAN there are also rdlic.abc.local as license server, a rdwa1.abc.local with the Webaccess service, a rdcb1.abc.local with the Connectionbroker and rdsh91.abc.local as a first session host.
- An officially signed wildcard certificate *.abc.de is installed for all servers.
- A RDS collection is created for a test user account.

What do work:
- It is possible to open the RDWeb from the LAN and from the Internet, access the RD collection and save it on the PC.
- It is possible to connect from a PC in the LAN to the rdsh91.
- It is possible to connect from a PC in the Internet to the rdgw1.abc.de using MSTSC by logging in with the administrator account of the rdgw1.
What don‘t work:
- Using the saved RD collection config on a PC in the Internet and connect to the rdsh91 through the rdcb1 and rdgw1. First the login credentials are asked for but then an error message appears: There are currently no resources available to connect to.
Therefor I am looking for assistance.