please post this question in the LR mobile forum.
At a first glance, I did not find an option to set.
In the camera view of the Lightroom Mobile App, you can do the following:
1. Tap the three dots in the upper right corner
2. Tap the Grid icon
3. Choose None, Halves, Thirds, Golden, and toggle on and off the level.
For more info: Get started with Lightroom for mobile app on iOS (this link will take you directly to the section on composing shots).