UNA is better than Dolphin in evert SEO-related aspect. However, UNA is not Dolphin and is not a direct replacement.

• If you switch to a new platform and want to preserve your search engine traffic you need to preserve content and URL structure, or at least preserve content and setup redirects for your old content to the new content. Eventually Google would remap it.
• If you look at your Analytics, check which URLs were the most popular and if you even have the same content still present on the new site. UNA has a different link structure and I'm not sure how much you've actually transferred from the old site - profiles, posts, comments, images - all that content typically attracts the long-tail organic searches in a community. If it wasn't transferred, naturally, you won't have that traffic.
• If you have't already, open your Google Search console and check if Google reports any indexing issues or manual penalties. In some cases there are issues that are easy to resolve once you know what it is.