I agree with L1ftmech. Should be a connector beside the air filter dirty/clogged sensor in which the original seat harness connects. See if someones jumper has jumped out.
I do not have access to an electrical schematic for this truck, but the fact that the monitor panel features a NEUTRAL lamp that begins flashing when the travel selector is moved and the truck does not move sort of screams out that the seat switch circuits are not being satisfied.
It is conceivable that the "OEM seat switch equipped seat" has been replaced with a cheepo seat without a seat switch and the OEM seat switch wiring has been monkeyed with to defeat that safety system, and now the monkey has fallen out of the tree.
Check the hood (bonnet) itself for the presence of a hole somewhere under the seat. The hole represents the seat switch wiring harness entry point. The hole will be visible only from the topside of the hood, hood insulation batt hides it on the underside of hood.
I hope somebody else here will be able to verify....or refute....what I have said.
But a NEUTRAL lamp on the monitor that acts like you describe says Operator Presence Sensing System in fault mode, the fault here being that it thinks the seat is not occupied.
this machine doesn't have a seat switch. Its been rode hard and put away wet, wiring every where has been cut or patched. Machine was driving then quit.
Check the seat switch wiring connectors for poor fir or corrosion.
There may be several connectors serving the seat switch harness on its way to the truck main harness and I have found that corrosion or a poor fit at any of the connectors can cause this.
A defective seat switch or switch operator bar/plate can give the same problem too.