@huyilin,

Double negatives = affirmative. So in this sentence, it means"I have tried this method, but it wasn't very useful" - Your understanding is correct!

e.g.我不是不想去(There are two 不, so this sentence is affirmative) it means"I'd like to go"

But Double negatives sentences are usually followed with "but…" or "just..." explanations as in我不是不想去，但是我真的太累了 or 我不是不明白，只是不想说。

我不是不想回答你的问题，只是之前没看见～～哈哈😄