It is incredibly important you only buy the specified OBD adaptor that is on the Bimmercode site. As a few have found, even one that looks similar will not work. The chap who made the app spent a long time testing various OBDs and he has found the most stable and reliable is the one he recommends.
Either do a search based on 'Vgate iCar 2 Wi-Fi OBD'. The top link takes you to Amazon to this one;
Vgate iCar 2 Wi-Fi OBD
Don't scrimp and try and save a couple of quid.
(I have no affiliation to Bimmercode or VGate, I just know getting the correct OBD in this case is a 'must'!)