If I click a wiki link, how can I go back in iOS?
In Bear 1.X, we are able to do two-finger gesture to navigate back and forth between notes. It hasn’t appeared in Bear 2 beta but my guess is that they should be back once 2.0 is launched? 
But to your question, there is no way doing that as far as I am aware of.
New history back & forward buttons < & > have just been implemented in build (11093) for Mac, so I think it’s coming soonish to iOS/iPad as well – It’s on their todo-list.
Until then, you could use the new back-link functionality under the (i) menu
(although it’s 2 or 3 taps away):
