There are several possibilities to find help listed at
https://www.krankenkassen.de/gesetzliche-krankenkassen/beschwerde/
You can call the helplines on Mondays to Thursdays from 8 a.m. till 6 p.m. and on Fridays from 8 a.m. till noon.
Concerning Health insurance: 030 340606601
Concerning Nursing insurance: 030 340606602
Concerning Health prevention: 030 340606603