Mixed topology

I have a building with one area containing one main line and several lines all linked in TP. Now I would like to connect that building to a new building via IP. The new building contains one area with one main line in TP only. The architecture I'm trying to implement is the one in the attached picture from the KNX basics tutorial. I understand that I need to set my IP routers to have physical address 1.0.0 and 2.0.0, but unfortunately, ETS is generating an error when I try to drag and drop my IP router at the area level: The operation could not be executed, because this coupler needs a main line with the medium type 'IP'. But I also can't change the medium type of my main line because it contains TP devices, just like in the tutorial picture. So I don't understand how to implement the architecture shown here.

Thanks for the help!