Wall heaters do not reach the desired temperature

The 21 degrees are measured correctly, right?

What do you mean by measured correctly? There is a thermometer hanging in the bathroom.

What do you mean by correctly measured? There is a thermometer hanging in the bathroom.

Have you checked with another thermometer?
Is it hanging on an exterior or interior wall?
At what height?
Have you measured in another spot?

The question is quite justified.

Perform a hydraulic balancing and don’t just turn any heating circuits randomly. This can ruin the entire hydraulics and nothing will get warm.

Possibly the heating circuit pump cannot deliver the required quantities.

Do you have individual room controllers? If yes, then disconnect them or open all of them and then perform the hydraulic balancing.

But maybe in the end it turns out that it's easier to put an auxiliary heater in the bathroom

He already has that with the wall heaters... What are the dimensions of the heating coils? Diameter and wall thickness? (Straws?)

Perform a thermal balancing and don't just turn any heating circuits. That can ruin the entire hydraulics and nothing will get warm at all. Possibly the heating circuit pump also doesn't achieve the necessary volumes. Do you have individual room controllers? If so, then disconnect them or open them all and then perform the thermal balancing.

Thermal balancing is definitely the right measure. The OP has throttled the other rooms and they still get warm. The bathroom has 3l flow in the underfloor heating circuit. That doesn't sound like there isn't enough flow available, does it? ERR the OP has probably already opened it up, at least that's how I understood it.

He already did that with the wall heaters...

Yes, I also find it strange, as written at 35 degrees design it should already work.