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).
Lightroom Mobile has a variety of grid overlays for the live camera view, but none for the crop tool? What sense does that make? C'mon, Adobe, get it together. How much manpower could it possible take to add these overlays to the crop tool?
please try to do a tap and hold inside or outside of the crop.
Let me know if that doesn't help.
Two finger tap locks the mode.