“The problem is I can't do it the other way round i.e. have the melody play in the sequencer while I practice the chords and have the APC playing. If I turn off the CHD track and press Start the melody plays but the APC will not start so I can't practice the chord progression”.

Any ideas how I can play the chords with the melody playing via the sequencer ?

The APC plays according to the chord track you record. If you turn the chord track off the APC has no idea what to play. The chord track speaks to the APC – when you turn the chord track APC cannot play.
I suggest that you play back the song you have recorded, make sure the left hand is turned off and play your chords with your left hand on the right side of the keyboard.
The best thought would be to play the song live and if your voice troubles you hum, whistle or moan. And please don’t take that seriously. Most of us just playing our chords live until we were comfortable.
Hope this helps, John C.