AWS S3 upload/connection fails with curl error 7

I started rebuilding an image uploader last night which uses AWS S3 for storage. I used composer to install the SDK, added a composer.json file composer install ran fine (have folders for aws, composer, guzzle, symfony).

Tried to then run an upload and got the following error:

Fatal error: Uncaught exception 'Guzzle\Http\Exception\CurlException' with message '[curl] 7: couldn't connect to host [url]

I did a bit of digging around and found various answers suggesting enabling openssl in the php.ini file, I've checked that curl is actually installed on the server, but it seems I can't make basic requests through curl at least according to this answer (performed the same test and it returns nothing, after timing out?)

I'm unsure how to debug this any further. Any help or suggestions would be appreciated.