External SSD no longer bootable
When upgrading from Sierra to High Sierra, the firmware on older Mac logic boards is also updated. This allows a Mac to boot the newer macOS versions (starting with High Sierra) from APFS container partitions. Sierra can not be installed to boot from a APFS container, but Sierra can recognize APFS and therefore allows the selection an APFS volume with Mojave as the startup disk. However, the firmware may not agree with the selection. Possible reasons for failure are given below.


The upgrade from Sierra to Mojave do not also update the firmware.
The firmware update did not occur because you used an external drive to upgrade macOS.


You could backup your important data to an external drive, then do a clean install of High Sierra to the internal drive. If High Sierra is not available, then select a newer version of macOS. Afterwards, you can do a clean install of Mojave to the external drive.

Update:

Back when High Sierra was first released, I remember many questions similar to your question were posted here a Ask Different. I believe the consensus was that the firmware can  only be upgraded from the internal drive. The update is written to the EFI partition on the drive before the firmware is instructed to update itself. The firmware is not capable of reading the update file from a USB drive. The firmware is capable of reading the update file from the first internal drive (disk0).