Translation:The entrance is not the door, but the window.
It sounds weird to me in English. I am not sure what is what in your sentence. The door is the window or the entry is not the door or ... it is confusing a little bit. How about we add one more little word to make it clear:
"The door is not the entry, (but) the window is."
That sounds acceptable to me, in English.
You can also do the same in the Hungarian sentence. Following the Hungarian logic at the same time:
"Nem az ajtó a bejárat, hanem az ablak."