I agree with JimDandy, the best way is to just connect your iPad to the TV.
If you go any other route here is what needs to happen
1) The incoming stream goes over your wi-fi to your iPad
2) The iPad then streams it back to the router over whatever app you select again over wi-fi
3) The router would then send it to the Roku over wi-fi.
That is a 3 times the amount of work and bandwidth you are expecting to utilize for your router. Some routers can handle it, others might not.