FIX 503 error (Varnix with Apache on CentOS - Magento application)

Facing 503 Backend fetch failed error. Here's how the environment looks like : Varnish 5, Apache 2.4, NGINX on CentOS 7 for Magento 2.3 application. It's a subdomain with a folder named as DEV eg: [login to view URL] We've tried implementing all recommended fixes including correcting ports, [login to view URL], daemon_opts. Also updated header length and response size in [login to view URL] to include below -p http_resp_hdr_len=65536 \ -p http_resp_size=98304 >>>>>>>>>>> Error 503 Backend fetch failed Backend fetch failed Guru Meditation: XID: 65567 Varnish cache server ->>>>>>>>>>>