to be able to use 3G for internet while connected to wifi? I have a wifi without internet which my iphone connects for a media player only. 
Meanwhile I would like to use my 3g connection for e-mail traffic etc. 

I have seen the answers to select the static ip  solution ; (I have an iphone 6) 

and yes it works if I connect to wifi at first , then change settings at the wifi connection to static ip and other things etc  , but I have to do it everytime I connect to this network,  

is it possible to have a kind of setting to do this once as a set instead of doing it every connection. if not it seems I will buy a used iphone 4 / simple android tablet to do this for me.
Thanks and kind regards
You have a new option from iOS 9,

Settings -> Cellular -> Scroll to the bottom of the page -> Wifi-Assist


  Automatically use cellular data when WIFI connectivity is poor


You have some articles about that all around the web
http://appleinsider.com/articles/15/10/14/apple-posts-ios-9-wi-fi-assist-guide-in-wake-of-complaints-about-excessive-data-use
http://www.macworld.com/article/2987233/smartphones/how-to-turn-off-ios-9s-wi-fi-assist-to-save-your-data-plan.html