Bootable usb ubuntu, unable to find in bios, change boot sequence on Dell laptop?

I installed full ubuntu on a usb-stick and it works but as soon as I’ve used windows 10 once I can’t find the ubuntu option in my bios anymore. I wondered if it had something to do with my boot sequence so I started to look for a solution here. I have a Dell latitude laptop. The options for my boot sequence are ‘legacy’ of ‘UEFI’. I’ve tried setting it on legacy and checking the ‘USB Storage Device’ but this didn’t help. Now in the UEFI option I can ‘Add Boot Option, here I can choose for ‘ubuntu’, looks promiissing, but than I have to choose between multiple options: fwupx64.efi, grubx64.efi, grub.cfg, shimx64.efi, MokManager.efi, mmx64.efi and fbx64.efi.

I’ve no idea what I should do or what goes wrong. Any help is welcome.