While I recommend looking through some of our troubleshooting guides to start, if your server isn't able to cURL external sites, you can always reach out by opening a Support Ticket since this can be caused by issues on the infrastructure.

Here are some guides that could help:

Troubleshooting Basic Connection Issues
Troubleshooting DNS issues

One thing that may help this situation is updating the DNS resolvers used by your server. Keep in mind that using Network Helper will rewrite any configuration files and set them back to our resolvers, so you will need to manually configure the networking to change those configurations.