So I've gone down a tiny rabbit hole because I wasn't aware of this issue (so take this with a grain of salt 'cause I only started an hour ago
). It sounds like submitting a DMCA take down notice generally solves the issuers. These sites don't know what is and isn't stolen until you notify them of it, and because of the DMCA notice they'll be able to look into what is going on. Many have received results from this, but it may take a couple weeks, sometimes even a month or more, because there's no computer automatically taking everything down, it's people who are dealing with millions of novels (also commonly working in a different time zone, with different holidays, days off, etc).
The DMCA thing isn't guaranteed to work, but when you do email them, you can at least wait it out and stop thinking about it. Give yourself a reminder for a month from now. Check up on them. Wait again. As others have said, it can be an endless battle, not just because of the slowness in taking these stories down but those who steal webnovels can just steal it again and post elsewhere, or on the same site after a few months.
1) https://www.dmca.com/faq/What-is-a-DMCA-Takedown
2) https://copyrightalliance.org/faqs/what-is-dmca-takedown-notice-process/