@Jayace 

Hi,

I just tested this with my Chromecast "3rd generation" (running the most current firmware version) and another router model of another brand and I can tell you that this issue is not specific to your TL-WR902.

If the Chromecast is being powered up and run without Internet connection, then it is basically unusable.

Apparently Google doesn't want us to use their Chromecast devices without them knowing. 

However, what I also noticed is that if my Chromecast did have an Internet connection when it was booting up, but I cut the Internet connection to my Wi-Fi router afterwards, then I still remained to be able to connect to the Chromecast and cast to it from my Android phone and my Windows PC (both also cut from the Internet). 

I must note that I haven't tested this for a prolonged time and I don't own any iOS devices to test this with.

But perhaps this is still something you could try out when you get a chance and also have the required equipment, even though your Chromecast hardware is different than mine. Power the Chromecast dongle up while it does have an Internet connection and then keep it under power via its USB cable continuously.

  @woozle So I did test this yesterday and as soon as I disconnected the internet ( I was using an iPad as the bridge for the travel router) the casting functionality stopped working from my iPhone to the Chromecast. I've read articles that say Chromecast will work with IOS devices if you have a mobile hotspot/ travel router to connect your devices. Which is why I think somehow the travel router is looking for an internet connection and preventing the devices from talking to one another.

It seems like if I can disable the WAN it may work.

  @Jayace 

When set to "WISP" mode (mode switch at "Share Hotspot") the TL-WR902 should establish its own sub-network that is independent of the WAN connection (the wireless connection to the iPad in your case). 

It would be extremely weird if the TL-WR902 prevented devices on its own "LAN" network from talking to each other when the WAN connection was disconnected.

You could verify whether a connection through the TL-WR902 is still possible if you "ping" the Chromecast device from your iPhone (there should be free apps available on the App Store to do this, I guess). You can see the IP addresses of the connected devices in the TL-WR902's configuration webGUI under the menu DHCP -> DHCP Clients List.

Another possibility might be that Google may have made changes to the iOS version of their Home App, making an active Internet connection of the iOS device mandatory for the Cast feature to work.

After all, Google updates their apps quite frequently, so an article written in the past might not necessarily reflect the present state of things.