I managed to get it working following the instructions of fis article: https://blog.ithasu.org/2018/01/using-ipega-9023-on-recalbox/

Hello
I am trying to make my Ipega pg-9139 controller with recalbox. It can operate in N-Switch or Android compatibility mode. I can pair the controller in both modes with recalbox, but that's it. When I am trying to configure the controller it does not react on pressing any of the buttons. I can't test the N-Switch mode as I don't have a Nintendo Switch at hands, but the Android mode works neatly with Android phones - and recalbox is told to support android compatible controllers.
What am I missing?