On PC, the two rankings are all separate:
- levels from diligence, collecting a lot of XPs to release equipment,
- tiers from skills (and equipment performance), by shooting low scores.
There, one can stay at any tier forever by avoiding certain points.
On mobile, as I said, there are much more options for "practice" rounds, so they introduced the influence of levels on tiers. Mobile players are promoted in tiers regardless of their scores.