Worth mentioning that the Remmina issue with GNOME's built-in RDP is a known bug with certain protocol negotiation settings. Try these in Remmina:
- Connection → Security → set to "RDP" (not "Negotiate")
- Under Advanced, disable "Network Level Authentication"
If that doesn't work, xfreerdp from the command line is more reliable:
xfreerdp /v:your-server-ip /u:username /dynamic-resolution
For a more robust setup, I'd actually recommend xrdp over GNOME's built-in — it handles multi-session and reconnection much better.