Hi there, we’re not familiar with Synology and don’t provide a Full-Text RSS package for it, so this might be a community-maintained package that you’ve installed. If so, it’s best to ask for support from either the people who maintain the package or people familar with Synology packages. (If anyone in this forum is familiar with that, hopefully they’ll chip in.)
Full-Text RSS is a PHP application, and typically installing it consists of placing all the Full-Text RSS files in the folder you want it run it, e.g. /full-text-rss/. Uninstalling would be removing that folder. But again, we’re not familiar with Synology packages, so there might be additional steps involved there.