All the best adblockers for iOS have automatic filter updating in the background. I'd like to see that come to Adguard for iOS. It's kind of a nuisance have to remember to open the app to update the filters.
Actually, it does check filters updates in the background. The only important thing to note - do not swipe Adguard from the list of launched apps as in this case iOS considers that you don't really need the app and does not let it run in the background.