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 030340606601
Concerning Nursing insurance 030340606602
Concerning Health prevention 030340606603