I would have thought your DX controller should automatically send the stick movements to the correct channels for Mode 1 when you've moved the slider switch to that mode. You could try reassigning the channels manually in the Select Controller dialogue box then click edit.
I don't know if you knew this but have you tried editing the transmitter channels?
Select Simulation from the menu bar at the top.
Select 'Select Controller'
I don't know what you are using as an interface but I guess that should show up on this dialog box.
There is a drop down preset profile...
Could be dirty pots in the sticks. I had a problem with glitching with mine so I sprayed the pots with alcohol spray (I don't have any switch cleaner). Both sticks seem fine now. That was a year or two ago now.