As a Master RA agent with a Swiss mobile number, I have simultaneously activated the Mobile ID SIM and app.

Why is the login to the RA app and the RA admin portal only via the Mobile ID SIM possible?

Mobile ID SIM has priority over the Mobile ID app for customers with a Swiss mobile number. We recommend choosing only one Mobile ID method and deactivating the other. You can deactivate the respective Mobile ID method in the Mobile ID Dashboard. 

Here is the manual for deactivating your mobile phone's Mobile ID.