The system, like most in the last few years, is UEFI-only -- it does not have a CSM/Legacy mode.  You are limited to operating systems that are UEFI-compliant. If you need a system that still has legacy support, you need to go back to a system a few years older than this one.  
I carried over an SSD with a non-UEFI Linux installed on it from another laptop. Many years ago when I switched from ASUS Rog Strix GL702ZC (all AMD, Ryzen 1700) to an Acer Predator Helios 500 Ryzen I just simply took out the SSD, plopped it into the other laptop and went on my way like nothing happened. I thought I'd pull the same trick moving to the m17 R5, but I cannot boot onto the SSD. The BIOS boot mode only shows UEFI and no other choice. On the boot page of the BIOS only the two network boot options are visible and that's only what the laptop is trying. That's where the BIOS should list the available bootable OSs / partitions. I tried both secure boot on and off. The main tab if the BIOS properly identifies the SSD so it seems it. The TPM is enabled in the Helios. The Linux is a Devuan. I'm using for 5 years now.
m17 R5, how to enable Legacy Boot mode?