https://wikidevi.com/wiki/TP-LINK_TL-WN821N_v4 shows that a version of TL-WN821N has a Realtek chipset (that needs nonfree firmware), so not every device labeled as it works with free software: buy it only if you are sure it's one of the first three versions. I have TL-WN722N which is bigger, I think it has no version with an unsupported chipset yet.
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