The old topic talks about the right thing, but is referring to old values for the setting. First, the not seeing Internet at all is not the same problem as there was recently where the 3D Warehouse window contents were empty. For people to see that issue, they absolutely did have to be online. What you’re seeing is that the online test is failing.
I have a theory about that, which you could help test. In older versions of SketchUp there is an i18n.dat, that amongst other things holds the URL that is used to test if there is Internet. The entry in the file looks like this:
ONLINE_TEST_URL=Google
It could be that your network has security settings that prevent applications from accessing an HTTP URL. So, edit that entry and change the value to be https://www.google.com
If that doesn’t work, perhaps Google is blocked. Try https://www.bing.com instead.
The file to edit is in here:
C:\Program Files\SketchUp\SketchUp 2018\Resources\en-US
You may find that you can’t edit and save in that location, but copying the file to your desktop, doing the edit, and copy/replace it back in the right folder, should work. You may need to enter an admin password while doing the replace.
The file is a text file, Notepad can edit it.
Please let me know if making that URL be https instead of http does help.