It’s keto breath, it comes from your lungs. There’s nothing you can do for it except not be in ketosis which means not fasting. No amount of brushing, tongue scraping, rinsing, or breath mints will help. Anyone that tells you otherwise doesn’t know what they’re talking about.
Make sure you’re hydrating properly and I would also recommend getting a tongue scraper, since the dead cells on the tongue aren’t removed along with us eating food, they tend to stay on it, producing smell/bad taste.