Allow me to step in, like Jamie said, the only way to install TeamCalendar in the "offline" mode is to upload the plugin manually. To do that you need to go to our Atlassian Marketplace -> https://marketplace.atlassian.com/ there you can type Team Calendars, and click on the 'free 30 days trial' then you'll be able to download a .jar file.
Once you have this .jar file, you just need to go to your UPM and click Upload Manually.
I hope this helps.
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