Install Ubuntu with full disk encryption on external SSD

I want to install Ubuntu with full disk encryption on an external SSD which I can boot from my XPS13. You can trick the Ubuntu installer to do this for you without having to go through the gruelling process of manually setting up the partitions and encrypted volumes by installing without any internal drives and just checking the disk encryption radio box. I have done this successfully on an old desktop, and I can boot the external SSD on the old desktop. Everything works as expected with FDE. However I cannot boot the drive from my XPS. Even when disabling secure boot and enabling legacy boot mode, it just won't detect the drive in the boot options. The desktop is quite old and doesn't have an EFI so there is no EFI partition on the external drive. I even tried removing the NVMe drive from my XPS13 and then doing a full install with FDE on the external drive from the XPS13, this added an EFI partition and asked me to setup a secure boot password, however the Ubuntu installer failed right at the end. Any ideas on how to get this to work? I have an external drive with an install of Ubuntu with FDE fully working, I just can't get it to boot from the XPS13.