Your former supervisor has represented your work as their own, which is the definition of plagiarism, regardless of whether this work was published before.
What you should do about it is a different story. You should consider your current position, your potential for future positions, and how an official complaint to an editor, or official recognition of your major contribution to the published work would impact these. Getting the credit may or may not be worth the trouble you start by bringing up the issue with an editor. Your first instinct here should be to avoid doing yourself or your career irreparable harm.
In the US in an academic environment, someone who reports misconduct (and we are talking about misconduct) has a modicum of protection, in that retaliating for a report of misconduct (at least scientific misconduct, and probably some other forms) is misconduct. This means that the institution is likely to exert strong pressure on those in a position to retaliate to NOT retaliate, in fear of liability. In China, I have no idea what the legal environment protecting whistleblowers is, but I would believe it is not as robust as in the US. Even with such protection, a whisper-campaign can be hard to prove.
There are also cultural and legal differences between my particular background and people in that part of the world, and some of these involve concepts surrounding intellectual property (see https://en.wikipedia.org/wiki/Intellectual_property_in_China for a bit of a review, and perhaps some good bibliographic material on the matter. You might also peek at Xu, Comparative analysis of intellectual property between China and the West: A cultural perspective, J. Intellectual Property Rights 2014(3)(http://nopr.niscair.res.in/handle/123456789/28926), https://www.researchgate.net/publication/235330028_'Plagiarism'_and_the_Confucian_Heritage_Culture_CHC_Student, and https://uselesstree.typepad.com/useless_tree/2010/08/plagiarism-and-confucianism.html. Beyond recognizing that there are differences, I'm not particularly well-qualified to comment on them, and would certainly welcome an additional answer that can expand on this. To my untrained eye, it's not just a legal issue, but it has something to to with the concept of intellectual property and how people think about it. You can't just make laws and expect people to follow them when the concepts lying behind a law don't match up well to concepts within a given society.
It would seem that as China is becoming more and more involved with the rest of the world, these issues are improving: https://foreignpolicy.com/2019/10/16/china-intellectual-property-theft-progress/