To find out that information I raised a general WHOIS query about ao3.life (via any sanctioned online WHOIS tool). The response which I posted to r/ao3 was quite trimmed down but contained the nameservers, which point to Starship. That plus a general search on DDG to learn who administers *.life took me to EuroDNS's page.

From there it was just carefully checking the corporate webpages for their forms on abuse report / copyright violation report.

That said, AO3 Legal team notified they are already aware of the issue. But they are volunteers currently being run over by the downtime and security attack, so I'd guess we should do our part to help by, say, mass-reporting the site. Any legal report can be of help, such as a DMCA takedown (if you fulfill the requirements to raise one), a trademark violation report, an abuse report, a C&D, etc.