Unable to fetch any information about available new releases and updates.

I have been having problems with update status for several days.
Cron is not able to run this & when manually done, I get the error message "Unable to fetch any information about available new releases and updates."
Furthermore, on the "Available updates" page, there is just this text:

Here you can find information about available updates for your installed modules and themes. Note that each module or theme is part of a "project", which may or may not have the same name, and might include multiple modules or themes within it.

To extend the functionality or to change the look of your site, a number of contributed modules and themes are available.

Last checked: 0 sec ago (Check manually)

No information is available about potential new releases for currently installed modules and themes. To check for updates, you may need to run cron or you can check manually. Please note that checking for available updates can take a long time, so please be patient.

and no single module listed.

This is happening on 3 different sites on the same hosting account.
According to some forum posts, it's because the update IP or host is being blocked by the host server.

My webhost says if I tell them the IP or host Drupal uses for updates, they'd unblock it.

So does anyone know whether or is this the explanation for the failure to get information on new releases & updates?

If so, what's the host/IP?

What about the behavior of the updates fetching process would lead to its being blocked?

I have not been able to update anything for over a week und would be grateful for any assistance to resolve this.

Thanks in advance.