I got a comment from John, he told me that a telco in Germany called Telekom has SP104 firmware out for B593 u-12. They seem to call it Speedport LTE II. The download link to the firmware is http://hilfe.telekom.de/dlp/eki/downloads/Speedport/Speedport%20LTE%20II/Firmware_Speedport_LTE_II_V100R001C748SP104.zip
The UI has English language, so it should be usable for us non-German speaking users too.
As usual, I did download and extract the firmware and all looks ok to me. Also as usual, I didn't test it. If you do, please comment how it went.
Update 21th May 2014:
Today John told me that, Telekom released a security update SP105 firmware. There is little information what the fix is, but you can get the lates from this link http://www.telekom.de/dlp/eki/downloads/Speedport/Speedport%20LTE%20II/Firmware_Speeport_LTE_II_B593u-12_V100R001C748SP105.zip