The fact that the engine starts with starter fluid suggests that there’s something wrong with the fuel pump system that is not getting fuel to the injectors in sufficient quantity to start the engine. Air in the fuel rail can cause this problem, so the first step is to bleed the injection system to remove any air.

Does the problem only happen when the engine is cold? If so, you might have a problem with the engine’s glow plugs. These are designed to pre-heat the combustion chambers to allow the diesel to ignite in a cold engine. But starter fluid can be a lot more volatile than diesel and doesn’t need glow plugs to set it off. Which could be why it will start on the fluid, but not normally.

From there, you need to look at things like the injectors themselves, fuel filters and leaks anywhere in the system that could be allowing the fuel pressure to bleed off between starts.