It requires your pihole to act as DHCP server as well. From what I can tell, Roku is hard-coded to use whatever address is handed out. It’s easily responsible for 60% of the blocked domains on my pihole.
If you don’t have admin access to the ISP router, your only recourse is to put a consumer router behind it (You’d just hook up the consumer router’s internet port to one of the LAN ports on the ISP router) and connect all of your devices to that. That way you can disable the DHCP server on the router and enable it on the pihole.
My router, (a TP-Link, can’t remember the exact model) will still list itself in the list of nameservers even if I just specify the pihole. Since I can’t seem to find anywhere in the router’s interface to turn that behavior off, I’ve resorted to using the pihole as DHCP as well.
But yeah, usually you can just use whatever DHCP server you have already