How long have you had this issue?
Sometimes hosts/isps have temporary blocks for whatever reason, which can solve themselves the next day.

See if you have any firewall/security extensions installed that might be getting in the way. Try disabling them.

I have had these reports in the past. Asking my host (SiteGround) whether the ip addresses have been blocked in some way always return the answer "No".
It usually turns out to be some firewall on the Joomla setup, some too-aggressive security module setting on the hosts server or some (temporary) blacklisting on some ISP.
In many of the cases, it seems to resolve itself after some time.

Are you able to install other (non-Regular Labs) extensions via the "Install from URL" method in the Joomla extension manager?

Can you give me (super) admin and (s)ftp access so I can take a look?
And tell me on what url I can see/reproduce the issue.
You can use the 'Hidden text' button in the forum editor to hide the confidential information.

Also, please contact your web host to find your outgoing IP address (different from your normal IP address).

Seems so. Happy it works again.