Offline plug-in installation process

1) Open JetBrains Marketplace in a browser;

2) Log in to a site;

3) Find a plugin e.g. CognitiveComplexity;

4) In the Versions tab, select a plugin version which covers the installed ReSharper build; 

5) If you are logged in, there will be a Download version icon:

6) Hit the Download icon and download the nuget package;

7) Put it to any local folder;

8) Specify such a folder here (Extension) | ReSharper | Options | Environment | Extension Manager | Add and uncheck .NET Products Gallery checkbox;

9) Run (Extensions) | ReSharper | Extension Manager and install the plugin which appears there.

Hint: if you use IE11 and it downloads *.zip file instead of *.nupkg, just change the file extension from "zip" to "nupkg".