Mobile redirect in nginx with query string
You are trying to do a negative match in regex to prevent rewrites when amp query argument is 1 in the request URL.
It is better to do this check with one more if block:
if ($arg_amp = 1) {
set $mobile_rewrite do_not_perform;
}
Then your rewrite can be:
if ($mobile_rewrite = perform) {
rewrite ^ $scheme://www.cnx-software.es$request_uri?amp=1 break;
}