Make an announcement thread here and be active on that thread giving updates on what's happening with the development of that dex project.
Have you tried leveraging social media platforms to your advantage? It's free and has a wide reach, so maybe you can try running giveaways or contests to attract attention to your project.
This is the first one that you should do aside from making an ann thread. Your social media presence is important and that's going to make you look conversational if you're engaging with the followers you have. So, you get to have an online presence from the forum as well as on social media. Since you've said that you're in low budget for marketing, it will take time until you build that organic followers and engagement of yours.