Using a WiFi router for ROKU TV and iPhone with-out ISP

I am looking for advice on how to configure a WiFi router with-out internet, in order for my iPhone to use airplay to watch streaming services on a ROKU smart tv. When I set up the router using the default settings, and try to use airplay the iPhone apps will say that I don't have a connection.

I read a suggestion on another thread that said to change the IP address from automatic to manual on the iPhone by clicking on the small i next to the WiFi connection. Then changing the IP address to 10.0.1.4 and sub-net to 255.255.0.0. I believe the suggestion was to let the iPhone know that this connection does not have internet. There must be a range of IP's that devices understand as not having internet?

I tried the above suggestion, but it did not work. I also tried to set up the router with the same IP address ranges. Does anyone have other suggestions on how to accomplish this?