It’s basic “how-to” when hosts in same local network.
So I would like build this workflow:

I used name “Server” PC and “Encoder” PC for my hosts.
“Server” PC
STEP 1: Download (link) and install the RTMPMiniServer. RUN IT.

STEP 2: press START
Note: if you got error message “Port 1935 not available” then change port in Settings

STEP 3: open Preview window

“Encoder” PC
STEP 4: Open any RTMP-encoder. I use OBS on macOS: go to Settings —> Stream: select Custom service and type server and stream key.

STEP 5: press Start Streaming and your encoder starts send RTMP feed to RTMPMiniServer. So let’s go to “Server” PC for check result.
“Server” PC (check result)
On “Server” PC I got this result:

Connect to NDI Workflow
RTMPMiniServer converts incoming RTMP feed to NDI.
So just open any NDI software (vMix, OBS, Wirecast etc) and select NDI source called “MiniServer A- Line 1”.
For example I use “NDI Studio Monitor” from NewTek tools:

