Hi, I never thought I going to answer my own question. anyway, I fixed the issue. this fix may help WordPress users/Developers deals with this same issue.
First of All This Mixed content issue caused by WordPress Default Sample Post Comment Emoji Icon. this emoji icon served from HTTP resources. just removing the demo post will solve this problem.
Plus I also Installed a Free WordPress Plugin Called “HTTP / HTTPS Removal” Just search this name in WordPress database and install/activate.
Finally, also don’t forget to Change URL’s in the Settings>General> Change Both Links to HTTPS.
These are some working fix checklists to try!
