I'd suggest using the pause button as a way to display a menu and in the menu allow the user to navigate back.
That way you don't clutter the game with unnecessary buttons and additionally you don't break the immersion while still giving a simple and intuitive way of navigate back.
Depending on the game it might be better to disable the usage of the back-button as the user might unintentionally tap the back-button and as such there might be a risk of quitting out of the game.
The Kingdom Rush games use this particular design to handle user navigation.
That being said it all depends on what sort of game you are making; does the user need to use both hands? If so, maybe it might be necessary to consider disabling the button as there might be a risk of unintentional button-clicks.
Is it some sort of flappy bird game where the user might be able to play it in portrait mode and only really need one hand? Than you might get by using the back-button to navigate between menus.
I'd also like to point out that you might want to use different ways of navigating while in different view-modes; if you are in landscape it might be wise to disable the back button or require a longer tap (click and hold), whereas if you are in landscape a single tap might be enough. So a more dynamic approach might be implemented too.