From your manual:
Step Four: Upstream cache
After all is said and done, don't forget to reboot the ISP's router to
flush its MAC address table, and in some cases a phone call to the ISP
to force them to flush their ARP cache is required.
The easy way is to send an icmp echo-request with the source ip of the
ip you added with destination your ISP's router. In this way the system
will relearn the new mac immediately