serve different content according to header value in Nginx

I have some web content behind the follwoing arch, for which I need to serve different content for desktop and mobile clients.

client > AWS Cloudfront > Nginx > ...

cloudfront has the builtin ability to identify the user-agent and they kind of unify it into 4 special headers (for example CloudFront-Is-Desktop-Viewer) which are either true or false.

on Nginx I'm trying to decide which content to serve according to those headers. for example:

 location / {
   if ($http_CloudFront-Is-Desktop-Viewer = true) {
        proxy_pass http://upstream;
        break;
   }
   root /var/www/static/en-US;
   try_files $uri /index.html;
 }

so in the above case it should go to the upstream if the CloudFront-Is-Desktop-Viewer value is true, and get the static files from nginx if it's false or not present.

but for some reason I always get the static files from nginx. I'm sure this header is being forwarded, I've even tried to send it directly from chrome with a header modifier.

what am i missing?

many thanks