Is there any app to play Bang! (Wanted) online (even solo or multiplayer) ?
Yes, you can play Bang! on Board Game Arena. BTW, there also are plenty of other board games.
There was an old ios app that is no longer supported due to the developer no longer having an agreement with the card games developer. After ios 11 the app can no longer be used due to it being a 32 bit app.
There is a thread on board game geek where there is talk of a new app in development but from what I can see that has been going on for a while with no end in sight.