no boot entries in Wyse 7020 (Z90D7)

I tried to install linux on a Wyse z90d7 (exact model on the tag says 909721-43L) and accidentally deleted almost all boot entries, including the one that goes into the setup menu. Unfortunately the one entry remaining in the boot menu intended to boot linux is wrong, and thus I can't change anything. Pressing just lands me in the same boot menu with the one faulty entry.

Does anyone know how to fix this?

I already tried pressing almost all the keys on my keyboard (all letters, function keys, del, home, end) as well as booting with a usb stick that had a UEFI shell at /EFI/BOOT on a valid esp partition. I also tried removing the CMOS battery. None of these appeared to do anything.

Is there a way to reflash the firmware somehow without using the boot menu?