What are these switches that are meant to move horizontally?
Last time I had one was some years ago in Sweden, and it was pretty straightforward I guess. Now, in Germany, I am not sure if am taking the heater's full potential.
These are standard radiator thermostat valves. They are usually marked
Note these are target room temperatures. Regardless at which setting you put the thermostat, if the air around the knob is significantly cooler, the heater should get so hot you couldn't touch it.
If the heater doesn't warm up sufficiently, there may be air in the system. Ask the landlord to blow it off. Also, if this is a seldom used holiday flat, the boiler may still be set to energy saving/summer. Usually, there is a thermosensor on the outside of the building so the boiler can tell the four seasons automatically but who knows what the landlord has set up.
If you have additional heaters in the room, turn them off. This will make the towel heater get warmer. You could even open a window and the thermostat will make it fight against the cold.
EDIT: I was told the question was about the sliders marked with a blue and a red line respectively. These are mechanical stoppers. They can be fully removed and put into another notch of the valve head. They are meant to show what are reasonable settings for this valve, and are often glued into place in public spaces, so people couldn't play with the valve setting —at least not full range—.