Paternity

Your question isn't clear. What exactly do you want to do, establish paternity, or change a childs name. These are two different things and should not be confused.

There are so many variables here but I cannot believe that the commune where the child is living will not have to be involved.
Where was the child born and the birth registered?
Was the father's name declared when the birth was registered? If not, why not? And, if the mother chose not to name you, how do you prove that you are the father?
Why do you think that the father not being registered in Belgium affects the child's surname from the time of her birth? (My daughter was born in Germany. I was not registered there but she had my surname from birth until she married.)

You would have to prove that the law has not correctly been applied. You would also need to know what the law is in the first place.

Pay a lawyer if it's that important.