Transmission problem

Hello, I have a caprice classic from 1985 Diesel 5,7 liter 105ps Since recently the automatic transmission switches only into the 2nd gear. The pressure box on the transmission is connected and works, What can it be ? Here in Bavaria no workshop knows itself. Are there any switch plans? Is there a cable missing? Because the oil has already been renewed without help. Various professionals say such a transmission does not break. It remains a mystery. Before I now thousands of euros in various repairs inves I would like to know if anyone knows the problem?