To enter Fastboot, one has to pus Volume UP and hold it, then power on the device while pressing volume up until the bootmenu appears.
UBports is buggy at this step IMO, but once you enter fastboot manually the flashing begins. Only after the tablet is in fastboot does UBports tell you how to activate it.
So for anyone reading this in the future, download UBports, enable Developer mode on the tablet (Ubuntu Touch doesn't have OEM locking), connect, then when the tablet shuts down, boot into fastboot and it should continue.