Configure ModSecurity to work with cloudflare

when I turn on the plesk ModSecurity I get an error 521 when I try to call my domain/site. I use the “OWASP ModSecurity” free subscription under plesk.

There are app. 30 to 40 rules which can be turned on or off. They have names like Capec-242 and I would need to know which rules I can turn on (and which off) to work with cloudflare.