The method that @Elusien suggested was the way to do it before Markers which just made their debut in Shotcut two days ago in version 21.10.31.
The new way to render just part of the timeline is to create a range marker. Set a marker on the timeline where you want to start the section you want to export. Then extend the end of the marker up to where you want to stop rendering. You do this either by holding Ctrl or (command on macOS) and dragging the marker up to where you want to or by right clicking on the marker, picking Edit then typing in the time you want it to end. After that, go to the Export menu and in the From category choose in the drop down menu the name of the range marker you created and now you will be able to export just that section. 
More information about Markers here: