Error: Cannot retrieve repository metadata (repomd.xml) for repository

Hi all,
I've inherited some AWS EC2 linux systems to patch and maintain and am having some issues with connectivity to the yum repository. I've done a bit of trawling before posting this as yum connectivity issues are quite commonplace but am still scratching my head.

The problem
I have four linux EC2 instances - all are in the same security group and share the same NACL.
They are private, so have no public IP and are reached via tunneling through from a Bastion.
The Bastion has internet access and has no issues with pulling down updates.
One of the four instances connects and pulls updates from yum fine and the other three chuck out this error;

http://public-yum.oracle.com/repo/OracleLinux/OL6/UEK/latest/x86_64/repodata/repomd.xml: [Errno 12] Timeout on http://public-yum.oracle.com/repo/OracleLinux/OL6/UEK/latest/x86_64/repodata/repomd.xml: (28, 'connect() timed out!')
Trying other mirror.
Error: Cannot retrieve repository metadata (repomd.xml) for repository: ol6_UEK_latest. Please verify its path and try again

I cannot see any configuration differences between the four servers. I've checked iptables and all seems fine. I've run yum clean all, etc..

What am I missing here? If all of them weren't working I'd feel better about this, but the fact that one can update from yum and the rest can't is throwing me! I've been scratching my head for some time, so much so that I have to actually post a thread here to ask for thoughts and ideas. It's going to be something obvious but I just can't see it.

Any advice would be much appreciated. Thanks :)

Edited by: Peejay996 on Mar 27, 2020 4:57 AM