I noticed that since I upgraded my Iphone to IOS27 I started to get ads on some of my news apps (New York Times and The Age Newspaper).
It turns out that the setting "Connectivity Assist" will use the cell provider to boost speeds on wifi. So even if you have Pihole set as your DNS on your phone, this feature will override it and you may see ads.
Toggle it off and restart wifi and the apps and they should stop.
Source: r/pihole · by /u/Enough_Survey_9404