Most RME recording interfaces have indeed such a button "Instr" to switch the channel to a higher inpedance to act as Instr/Hi-Z input.
If you consult the manual, then the analog inputs AN3/4 of the BBF Pro have already such a higher impedance per default, so that you can skip this point.
All that remains is to click to the wrench symbol of this input and to adjust the input gain level like you do with your mic inputs.

@RME: would be useful to detail this in the manual and to add a picture of channels AN3/4 where the gain knob can be seen. For customers who do not know such details it might be too difficult to come to the proper conclusions and I also had to look closely whether its correct what I am typing.

So you need base knowledge how it works usually, best need to know that Instr inputs have higher impedance and if you look then into technical data section of the manual, then you see that Instr 3/4 has already high impedance, so that an Instr button is not needed, etc. All very logic, but if you are new to it and head from other cases that an Instr button needs to be pushed, well I can understand that this causes a little confusion