Help! Who can explain how the circuit is handled?

Hello together! I made my CE driver’s license on an Actros with Telligent circuit about 2 years ago. Now we have recently purchased a MAN which however has another circuit. It is a 16 gears so 8 gears and then a switch at the switching knob to switch the groups. If I insert a gear and let the clutch come he throws the gears out again and again. After several attempts he suddenly left me in the 4th gear. In the 4th but NOT a chance to get into the 5th. Can just switch half a gear with the lever and that was it. I drove him already once and could drive without problems however I did not touch the switch lever. I can not imagine that something is broken because the truck was delivered a few hundred km over the highway. Vehicle is a MAN FE year think 2002. What am I doing wrong?? Can someone really idiotically This circuit is absolutely new to me! The circuit is as shown on the picture, however, with a side slider as a switch.