iMac Mid 2011 App / iTunes Store Problem

Hi,

I am trying to connect to the App Store on my iMac Mid 2011 with 12GB RAM and it keeps coming up with "Cannot Connect to the App Store - An Internet connection is required". The Internet is fine as I am currently typing this, but it is also not connecting to the iTune Store and coming up with the message "iTunes could not connect to the iTunes Store. An unknown error occurred (-9836). - Make sure you network connection is active and try again."

I have tried everything on this link

If you can't connect to the iTunes Store, iBooks Store, or App Store - Apple Support

But as I can't get on the App Store, I can't upgrade the MacOS so tried everything on this one as well

https://appuals.com/cannot-connect-to-the-app-store/

But no luck!! From what I've read in various forums, hopefully an upgrade to MacOS Sierra would sort the problem, but I can't find a download link that doesn't go to the App Store. Does anyone have one? or any other solution they could suggest?

Thanks in Advance

A very frustrated, Dan :-/