There's one other option which is askgamblers:

https://www.askgamblers.com/gambling-news/ag-news/agccs-is-now-accepting-sports-betting-complaints

https://www.askgamblers.com/sports-betting/sportsbook-reviews/stake-sportsbook

Its weird because the guidelines says it can't be a sports-related issue, but the first link suggests otherwise.

Anyway, if I'm wrong, I'm sure someone will correct me pretty quickly.