FL Studio 20 is a very sophisticated DAW, but the NetVST plugins are very simple. They work together, but you must use FL Studio's “fixed size buffers” setting.

After opening e.g. NetSynth on a track in FL Studio 20, before connecting, click the gear icon in the title bar of the plugin window as shown below:

This will open the settings page for the plugin. Click the “Processing” tab as shown:

On the Processing page, activate “Use fixed size buffers”. FL Studio will pop up a warning message; accept it and continue.

To return to the plugin's own GUI display, click the plug icon at the top left:

At this point, you can start network connection and the plug-in should work correctly.

You should stick with 44100 Hz sample rate, and keep your buffer size at 512 or 256.