Remotes and joystick are two separate control systems, easy option is to tape the remote to the chair arm till you get it sorted so it’s not forgotten
in some of the 420’s there is a circuit diagram stuck to the inside of the seat interface covers, you should be able to follow the control circuit through using that and test it to see where the issue is?
possible loom failure? The wires fail with all the flexing when you fold the arms up and down or swivel the seat
also check in the downside arm for a switch? Some lifts had a downside arm switch that meant you had to have the arm folded down for the joystick to work, normally fitted when the control arm was on the upside, it was fitted so that you had to have both arms down to stop you sliding off the seat! Mind you that’s what the seat belt if for but how many users actually used them!
solus and sierra seats had seat pad switches, two switches in the base of the seat operated by two pips on the chair, these wear away over time, the seat casting is much softer than the steel pins they push down on
but from your description of the joystick it sounds like you have a Saxon chair? so start with the chair loom and arm switch
