-Solved-
We recently had to upgrade server to add OJS3.1.2.4 which is when the error occurred. Our IT team fixed this accidentally - this is what they said…
"check that you can make a connection to Crossref by pinging or doing a curl first. In our case it was a previous proxy server setting (found in the config.inc.php) that wasn’t required and actually prevented the OJS sites from communicating out to the internet. As the server already has a direct connection to the internet, removing the proxy setting allowed the OJS sites to communicate properly again.
Jut found this in the PKP forums with similar suggestions:
Issues with the DOI-CrossRef automatic registration"