A dead internal backup battery is not the most likely cause on a D7100. Cameras like this typically rely on the main battery, and may use a small capacitor to preserve settings briefly when the main battery is removed.

If the camera is left without the main battery, or the battery is very low, settings may not be retained properly. Aging of the internal capacitor is possible on an older camera, but if that were the issue, keeping a reasonably charged main battery installed should usually prevent the problem.

So first check whether you are leaving the camera without its battery, or storing it with a low-charge battery. If it still loses settings even with a charged battery left in the camera, then an internal power-retention fault such as a failing capacitor becomes more plausible and may need service.

Also note that if settings change while shooting, some may be affected by camera modes or features rather than memory loss, so verify that behavior separately from true power-off resets.