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