I have used the Let's Encrypt in the past. However, it is not working now with either Chrome or Firefox browsers. I'm also using a no-ip DDNS when using OpenVPN.
If anyone finds this via search... running the standard Asus firmware, I just had to change it to "Import your own certificate" (which created a 20 year expiration local cert) and back to Lets Encrypt for it to show up properly now. Before it was just showing Status: Authorizing... (but actually was creating a cert, I noticed)