this is very specific for content on german public broadcasting. but if you, a law-abiding, tax-paying german resident find yourself on vacation and want to download geoblocked content from the mediathek, do the following:

-go to https://mediathekviewweb.de and find the url of the file you want
-copy-paste that file into a german web proxy. you can find lists of these online with your favourite search engine. might take a while to find a good one, but often there is.
-your browser will most likely show you the video immediately, you'll need to pause it, hit the alt key and use file -> save as to save the video file.