The first thing you can do is to ask the publisher/journal editor(s) (wherever the paper processing is with the journal in question) to make the addition. Since you are not the first author (assuming first = corresponding) you should probably go through that person. The likelihood of getting such changes done if the paper is published online is uncertain but worth asking about anyway.
Having your funding not acknowledged in the paper is not the end of the world so if changes cannot be made, well there are probably more papers coming later. It is of course always a god way to show the funding source where their money ended up. There might be funding sources that request such acknowledgements but there is no law. some authors never acknowledge anything. The fact that you are not first author is not terribly relevant. Any funding source adding to the total funding of the study is of importance to the results; the paper.
So in the end, I would not worry too much about it. It was an omission, a mistake; it happened, you make an effort to rectify and you will likely remember it in the future.