HaProxy giving - 503 Service Unavailable

Of course try the connection to backends yourself:

openssl s_client -connect 192.168.192.173:8443

This way you'll remove most of possible causes.

What if s_client works, but haproxy doesn't? On SELinux enforcing (for example CentOS 7 with default settings) exactly this happens and you need to explicitly allow haproxy to connect to any backend port:

setsebool -P haproxy_connect_any 1