A data broker is going to correlate this IP with "never gonna give you up" as an ideological statement about his drug dealings. They'll be receiving weird ads for weeks!
He's most likely just not very smart.
I looked into it and so far as I can tell it works off a blacklist system, rather than any sort of automatic analysis (eg. TCP or MTU fingerprinting). If you set up a "residential proxy" in the form of a home VPN, it won't be detected. It also means the detection is only as good as whatever their backlist source is. If it's a niche provider, it might not get picked up at all.
Where are you finding free residential proxies?
> Tried my CGNAT home connection - flagged. My phone connection - also flagged.
Why does that mean they're doing a bad job? Since both are CGNAT, you're sharing the IP with lots of other people, and it's not unlikely that one of your network neighbors is infected.