You could use nmtui and set Cloned MAC Address for your connection.
After that and before the reboot you have to edit your connection file, which is usually:
/etc/NetworkManager/system-connections/Wired\ connection\ 1.nmconnection
and remove mac-address line (not cloned-mac-address) from there as on next boot you physical MAC is different and Network Manager will not use the same connection file.
On the other hand, if you don’t need Bluetooth or Wifi, you could use Armbian image from here: Rock pi s debian ftdi driver missing - #12 by piter75 as it has patched u-boot with stable MAC derived from CPUID.
PS. I am working with Igor to have Rock Pi S in Armbian 