I'm not familiar with Chelsea Yeah, honestly. There's a chance you've encountered a really niche writer within a specific fandom who's got a cult following on a forum or a private Discord, but if they're putting out exclusive stuff, it's probably locked behind something like a Patreon or a Ko-fi. You'd likely need to follow the author's socials to find the link. I tend to be wary of content behind paywalls for fanworks, though—feels a little against the spirit of things sometimes, unless it's for original fiction. Maybe Chelsea Yeah is a pseudonym on a site like SquidgeWorld or one of the newer, smaller archives that cater to specific genres? Could be worth doing a search across the smaller, independent archives that popped up after some of the big ones got purged a few years back.
I tried a quick search myself and came up empty, which suggests it's either very new, very private, or I'm completely misremembering the name. Sometimes authors migrate their work to exclusive spaces if they've had harassment or want more control over their audience. If you're desperate, maybe check Tumblr tags? Authors often use that to signal where their exclusive stuff is hosted, even if they don't post the full works there. It's a bit of a digital scavenger hunt, and honestly, half the fun—and frustration—is in the hunt itself.