Hi there.
here is a simple way to check e.g. which MAC address has been configured in synoboot.img grub config without the need to mount them.
Linux:
Open a terminal, navigate to the folder where you have unpacked/stored the synoboot.img file and enter
strings -t d synoboot.img | grep -iE "set ?mac"
Windows:
Open a console, navigate to the folder where you have unpacked/stored the synoboot.img file and enter
findstr /c:"set mac" synoboot.img