My tablet runs the latest nightly of cm-13 and twrp. However, a bug in twrp doesn't let me update cm nightlies. So wherever I try to story my nightly I can't access it from twrp. Root doesn't work either for some reason so I'm stuck with a tablet that runs cm-13 but without updates.
After investing some time I tried connecting the tablet to a laptop while in recovery mode. Plugged in a USB cable on both sides and voila I had access to a twrp folder which was also mounted in recovery mode. Copied my cm-13 file there and was successfully able to install the nightly.
Connecting a USB thumb drive didn't work either btw.