Unable to open Constant Contact (r20.rs6.net) links on iOS devices

I've seen several posts where people couldn't open a link received in an email, usually when the links were created via Constant Contact. One main DNS name used by Constant Contact is r20.rs6.net. The error is usually something along the lines of being unable to resolve the domain name and will show in both Safari and Chrome.

Constant Contact is used to inject ads into the site you are visiting, so it is probably an add blocker that is causing the issue. Simply disabling ad blockers in your browser may work, or it might not. If it doesn't, then another solution you should try is to check your router to see if it is providing ad-block support. For example, if you use an eero router with eero Plus support, it will, by default, block ads from sites, which will keep you from being able to resolve the Constant Contact sites. Disabling this ad-blocking support should resolve your problems; it did for me after spending way too much time investigating why it would work on my MacBook, but not my iPad or iPhone.