Problems fetching items behind local proxy

I can’t get it working to fetch the items behind a proxy.
I can get local RSS-Feeds, but extern Feeds are showing the error ‘Failed to fetch the RSS feed. Error: fsockopen error: php_network_getaddresses: getaddrinfo failed: Name or service not known’. I already tried setting the proxy in wp-config.php with ‘define(‘WP_PROXY_HOST’, ‘IP’);
define(‘WP_PROXY_PORT’, ‘Port’);
define(‘WP_PROXY_USERNAME’, ‘ProxyUser’);
define(‘WP_PROXY_PASSWORD’, ‘ ProxyPass’);
define(‘WP_PROXY_BYPASS_HOSTS’, ‘localhost’);’, but it isn’t working.
Thanks in advance

https://wordpress.org/plugins/wp-rss-aggregator/