You need to specify a channel, example: nebula?channel=Mustard
Go to the channel you want to subscribe to and look at the address, replace https://nebula.tv/ with https://leonick.se/feeds/nebula?channel=

This page uses publicly accessible APIs to get the latest videos of a given nebula.tv channel and generate a rss/atom feed.

As of April 30th 2022 Nebula finally have official RSS feeds, you can read more here.

That said, I have no intention of taking down my verion so long as it works, so feel free to keep on using it.