Just happened to me this morning. An automatic update of iOS for some reason switched off AdGuard-- I'd forgotten just how bad the ads are. Anyway, easily fixed by switching the parts back on.

iOS? Do you have "advanced protection" enabled? Since some time ago standard content blocking rules are not enough and we need a WebExtension to be allowed to run on YT to block ads there.

Oh and one more thing: there's an option to "share" video from the YT app or YT webpage to AdGuard. This is probably the only possible way to get rid of ads when you're using the YouTube app.