Common English Errors: When you talk about a situation in the past when you tried harder to achieve something and succeeded in it, then you usually use 'was able to' or 'managed to'.

Is not able grammatically correct?

There's no difference in meaning between "unable" and "not able", but they're not quite interchangeable in some situations. One difference that comes to mind is that we can modify the adjective "unable" with an adverb in ways that wouldn't work with the phrase "not able".

Was not able or could not?

We use could for general ability. However, when you want to say that somebody did something in a specific situation, use was/ were be able to or managed to (not could). For example: There was a boy in the river but people were able to save him.