In the iOS app you'll see a small floating ‹ button at the bottom-left on sub-routes (post detail, settings sub-pages, campaign detail…). It's not there on top-level tabs — dashboard, composer, posts, calendar, connections, analytics, settings, help — because those are the roots of the tab bar and going "back" from them doesn't make sense.
Tapping the button pops one route in history (same as your browser's back button on the web).
Once the native chrome bar ships in a future app version, the web fallback will disappear automatically.