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.