OK, so you use a MIDI track to play back MIDI data to the external keyboard, and It seems that the MIDI that you play back always has channel 1.
How do you produce this MIDI data? Do you load a .mid file? Do you use step-entry? Do you record it? Do you draw it manually with the “draw tool” and Mouse?
When you record your keyboard, Ardour will re-play the channel as-is. The same is true when you import a file, Ardour uses the channel from the MIDI file.
You can override the MIDI channel. The Easiest for the whole track (right-click on the track-header > Channel-selector…): http://manual.ardour.org/working-with-tracks/midi-track-controls/ “Use a single fixed channel for all playback”.
Step-entry (right click on the rec-arm button) http://manual.ardour.org/working-with-midi/step-entry/ has a “channel” selector. That should be straight forward. Selecting the channel when manually drawing notes gets is a bit trickier, see http://manual.ardour.org/working-with-midi/