Could you please check that your mbr, bios etc are all in agreement as to whether to boot UEFI, Legacy or otherwise?
For example, if BIOS is set to Legacy but mbr is set to UEFI, then it will prefer UEFI, resulting in the legacy mode boot option never appearing at all.
Can you check through each bit of this tutorial and see if anything looks like it may have been missied or could help:
https://linuxhint.com/ubuntu_boot_repair_tutorial/