For me, pressing the power button twice beings up a menu where I can choose which camera app to use if multiple are installed. In case you already chose a default app before, you can undo this in the app info settings
This could be, because I installed Lr only recently.
I am sorry, but in the App Info settings of the camera I cannot see how to undo this (“Standardmäßig öffnen - keine Standardeinstellungen festgelegt”), and for the power button there is no App Info …
I guess this because Lightroom Mobile does not register itself as a camera app. That is because it is a photo editor, which happens to have an in-app camera …