NAS326 Debian 4.12 rootfs Unable to set MAC address (no replies)

Hi all,
the method described in https://forum.doozan.com/read.php?2,32146 for setting the MAC address in /etc/network/interfaces does not work for me.
My router's webinterface shows a different MAC address for the box each time it connects, thus assigning a different IP each time.

My /etc/network/interfaces:

auto lo eth0
iface lo inet loopback
iface eth0 inet dhcp hwaddress ether 9a:b3:25:9d:b9:51

ethtool -P eth0
Permanent address: 00:00:00:00:00:00

ifconfig shows:

eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet 192.168.178.48  netmask 255.255.255.0  broadcast 192.168.178.255
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 792  bytes 149319 (145.8 KiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0
        device interrupt 36
        inet6 fe80::78c3:32ff:fe2d:c80d  prefixlen 64  scopeid 0x20<link>
        ether 7a:c3:32:2d:c8:0d  txqueuelen 532  (Ethernet)
        RX packets 2480  bytes 421784 (411.8 KiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 820  bytes 152867 (149.2 KiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0
        device interrupt 36

which matches the mac address shown in the router's webinterface.

- michael