I went through this before with the same model car. Your problem may not be the same, but I'll let you know what happened to me.
My wife bought a used 300D turbo. Everything was fine 'til I had to put a monovalve kit in and lost some coolant. There are two hoses from what you're calling the reservoir to the radiator. This is not really a reservoir, it is the top tank for the radiator just relocated. The top hose is small and goes to the top of the radiator for a vent between the two. The bottom hose is the main hose.
On her car, someone had removed the top hose and plugged it off. You could not completely fill the radiator. The plastic fitting for the small top hose was broken off the radiator. I used a metal valve stem that is used on aluminum wheels. It is threaded with a nut and two rubber washers. I put a piece of wire through the radiator top hose connection and through the small hole and pulled the fitting through, then put on the nut. I then put the vent hose on this and all was well.
I doubt that your vent hose is gone, but it should be clear.
As I said this is probably not your problem, but definitely see that the vent hose is clear.