It is important to understand that iOS is a fairly closed operational system. Apple doesn't provide many possibilities to block ads outside of Safari. The first thing you need to do is to enable DNS protection and choose AdGuard DNS server: AdGuard Settings → DNS protection → Toggle the switch to ON → DNS server → AdGuard DNS.
If it didn't help, do the following:
View the browser version of this app/service (e.g., YouTube, Facebook) in Safari. Since Apple allows Safari content blocking, we can block ads very efficiently in this browser.
Or please do contact firstname.lastname@example.org. We will provide you with further instructions on what can be done.