Thank you all for your help.
I had to find my ol computer screen which has a VGA (and also SCART) input to make test
Dreamcast -> Toro -> VGA -> computer screen = no signal
Laptop with VGA output -> Toro -> VGA -> computer screen = I got an image
So I suspect the cable to connect the Dreamcast to Toro. I opened it and saw 2 removed cables (I hope it isn’t my fault when I opened the sheath ^^).
I asked the support where I should connet thes 2 cables.