That sounds to me like you have need a proxy setup. If your IT group makes you go through a proxy server, follow the instructions in this other post for changing the proxy settings (in case it’s not clear over there: you must Run as Administrator to change proxy settings). I think this should fix your download issue
Your second issue (“even when I try to download and install from a third party”) was clarified in your other post in another compare-plugin thread (please keep your issue consolidated here) is because you “moved it into notepad + +\plugins”. That’s wrong, and has been for over a year (since v7.6.3 in Jan 2019). Notepad++ now keeps its plugins in <installdir>\plugins\PluginName\PluginName.dll. So the ComparePlugin would go as <installdir>\plugins\ComparePlugin\ComparePlugin.dll, with any subfolders it wants as subfolders of that folder (<installdir>\plugins\ComparePlugin\subfoldername); there shouldn’t be any DLLs directly in <installdir>\plugins anymore.