To fix the FitBit app using PiHole, add www.google-analytics.com to the whitelist.

If a group is created in PiHole, the FitBit, and the phone can go on the group you create for this.

I may need to work on this more, it works on my cell service, but not the wifi with PiHole using this setup.

Well, what I had to do was just allow this domain into PiHole for Google, add this domain to the PiHole whitelist.

google-analytics.com

Obtained from here: https://github.com/GoodnessJSON/PiHole-Whitelist/blob/master/lists/optional-list.txt#L103

Guide I found to help out with this: https://www.reddit.com/r/pihole/comments/hxlb4s/fitbit_app_and_pihole_issue/