I've got an elderly family member who is very susceptible to online advertising.  Particularly supplement scams and spam and he ends up spending thousands a year on junk supplements.  He lives overseas so my ability to help manage his network and devices is pretty limited, but when I recently visited him and observed his daily internet activity and the sites he visits, I can see he is getting targeted/pushed supplements/medical advertising at an unbelievable level.  I think the only way to protect him is to find a good ad-blocking option that really cuts down on the ads he is getting pushed, but doesn't disrupt his daily internet consumption.  Thankfully he doesn't use social media, but he does watch a lot of YouTube.

He's in the USA on a Spectrum internet account and I really can't easily do anything to configure and manage his router remotely so I'm probably best to look at client-side solutions for his Windows desktop and, probably his iPhone (although he doesn't do a lot on his phone and most of the damage is coming from computer based browsing).

Any recommendations here of what's the easiest, most stable (e.g. the adblocker doesn't start breaking important things like accessing banks, everyday youtube watching, news sites like NYTimes, power and utility companies, etc...) set-and-forget option? 

I'm kind of thinking it needs to be more DNS based.  Have tried browsers plugins like Adblocker Plus and uBlock, but those have done nothing to curb this problem and the script blocking on them occasionally interferes with normal and legitimate website functions.

The two that seem to come to mind are AdGuard and NextDNS.  I don't mind paying for a service as that will be far cheaper than the amount of money being lost to scammy health supplement advertising.