na-adjective + noun: can I negate na-adjective in this case?

Question

I read articles and watch videos but no one mention it is possible to negate na-adjective if it before the noun.

Everywhere just mention that case when no noun after the na-adjective:

これ は かんたん です。 --> これ は かんたん では ありません。

I know we make na-adjective from noun and if noun isn't after the na-adjective (as see my example) just simply negate noun without な.

But how negate na-adjective which is before the noun?

これ は かんたんな もんだい です。 --> ??? (maybe: これ は かんたん では ありません もんだい です。)

As I know this: これ は かんたんな もんだい では ありません。negate the noun (もんだい) and not the na-adjective. I know the alternative solution is put the noun before the は: この もんだい は かんたん では ありません。

I just ask this because I know it is possible negate i-adjective which is before the noun.

Answer 1

Yes it is possible. Perhaps it's not clear to you because you are using the formal form, ではありません, and that form cannot be used to modify nouns.

If you were to change to the informal ではない or じゃない then things start to look rather like the i-adjective case, right?

これは簡単{かんたん}じゃない問題{もんだい}です。
This is a problem which is not simple.