The MIDI channel for the instrument channel is set by right-click on the channel, then Channel->number
The MIDI channel on the Kontakt instrument is set as Mario indicates above.

I can "Save Group" and "Load Group" from the DirectX/VST Window, which speeds things up, but as above, it isn't saved in the .SEQ file even though RB thinks the song has changed.

RB seems now to have saet Kontakt as the (default) synth, so other MIDI instruments no longer play. That was why I had unchecked MIDI Drivers->Re-Route MIDI playback to default Dxi/VSTi Synth, as per here: https://www.pgmusic.com/forums/ubbthreads.php?ubb=showflat&Number=636746&Searchpage=1&Main=88211&Words=vsti&Search=true#Post636746
Unchecking that stops Kontakt playing.

So at the moment, I'm rather back where I started.

Jazz relative beginner, starting at a much older age than was helpful.
Kawai MP6, Korg M50, Ui24R, Saffire Pro 40.
AVL:MXE Linux; Windows 11; Win8.1: Scarletts
BIAB2022 UltraPAK, Reaper, a bunch of stuff.