If I were you.... I would find an old version of Firefox (like version 6 or 12...) and not let it upgrade... and connect with it to the ZD. The ZD software must be very old and only support deprecated crypto. You may also have good luck with an older version of IE with all its security options disabled. After successful connection, upgrade the software in the ZD following the supported upgrade path.
I am not aware if you could upload newer firmware and start the upgrade via SSH, but it's worth checking.