Diesels just need heat and fuel to run
You either have not enough heat or not enough fuel
Heat is provide to a cold engine mainly by the intake heater(on a 24v Cummins) if it’s working the intake manifold will be noticeably warmer than the surrounding engine parts.
And the compression from cylinders doing their up and down thing fast enough. Batteries are good and spinning the engine over fast enough?
Btw don’t use ether with an intake heater bad things can happen.
As far as fuel goes there are plenty of reasons it might not be getting enough.
Since it may have sucked air into the lines that is a good place to start because any air bubbles in the system can cause it to not start or run roughly.
Work methodically to verify where fuel is and is not. You can start by trying to prime it as a first step and see if that helps
Then you could move on to cracking the injectors to see if there is fuel there or not