Cannot ssh into altssh.gitlab.com on 443/TCP from RHEL/CentOS 6?

Need help to connect my on-premise deployment server to altssh.gitlab.com

ssh-keygen generated private-public-key-pair for 2 servers as gitlab & gitlab.pub files in ~/.ssh,

Public keys installed as deploy keys in gitlab.com on my project’s repository settings.

~/.ssh directory and file permissions are 700 and 600

created a ~/.ssh/config

Host gitlab.com
  Hostname altssh.gitlab.com
  User git
  Port 443
  RSAAuthentication yes
  Protocol 2
  PreferredAuthentications publickey
  IdentityFile ~/.ssh/gitlab-pub

Does not work on my actual deployment server :

RHEL 6 server (with the latest openssl and openssh* packages)
[splunk@svLDSSPLUNKPRD01 ~]$ ssh -Tv git@gitlab.com
OpenSSH_5.3p1, OpenSSL 1.0.1e-fips 11 Feb 2013
debug1: Reading configuration data /home/splunk/.ssh/config
debug1: Applying options for gitlab.com
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: Applying options for *
debug1: Connecting to altssh.gitlab.com [35.190.168.187] port 443.
debug1: Connection established.
debug1: identity file /home/splunk/.ssh/gitlab-sga type 1
debug1: identity file /home/splunk/.ssh/gitlab-sga-cert type -1
ssh_exchange_identification: read: Connection reset by peer

However does work on a test server :
RHEL 7.5 server

[splunk@svcmsplunktst01 ~]$ ssh -T git@gitlab.com
Welcome to GitLab, @jmut!

I cannot use the test server, I cannot upgrade the deployment server. I need to get this to work on RHEL 6.

Thanks for any help you can offer.