SSL redirect errors with Cloudflare for subsite


I am trying to set up HTTPS for a subsite in my network. I have two certs able to be used: one is a Let’s Encrypt cert on the server where I have set up domain mapping (the domain is added here as an addon domain) and the other is the free SSL cert that Cloudflare offers.

The issue is that after setting up the standard page rule for HTTPS, I get the redirect error that one often gets with WordPress and Cloudflare, no matter which cert I try to use, and no matter whether I am using Flexible or Strict settings in Cloudflare. I have installed the plugin called Cloudflare Flexible SSL by iControlWp, which I have used successfully on many single sites, but the settings window doesn’t seem to appear in the dashboard of my subsite. I have filed a support request with the plugin designer to find out if this plugin is multisite-compatible or not.

Is there another plugin that is recommended for use with multisites? How do other people set up SSL for their subsites?