Regarding method 2, can i install Cloudflare Flexible SSL, if my site traffic is already going through Cloudflare?
Regarding method 1, i don’t know how to add page rules. Where can i find more info for a complete newbie?
You shouldn’t be using Flexible SSL. Your host should have an SSL certificate for your domain on their server.
Try using your browser’s Dev Tools (F12 in Chrome) and check the Console and Network tabs for errors.
I don’t think that Page Rule to “Ignore Query String” is going to help, as that’s a minor tweak of caching settings. But you’re welcome to try adding a Page Rule in the Cloudflare Dashboard:
Match *example.com/wp-admin/* and add setting for “Cache Level: Ignore Query String”.