Have you checked YouTube? I'm sure this isn't the first time this question has been asked and if there's a how-to you'll find it on YouTube.
Are you just trying to display your screen or are you trying to play YouTube or some other app? What are you wanting to broadcast? Again looking for an app specific answer vs. just how do I, is more likely to find you an answer.
In the past I've had issues connecting to my Smart TV when my phone is not on the same network as the TV. Do you have multiple WiFi networks? I have 3 and it seems to be the norm to have at least 2, a 2.4 and 5 ghz. Are your Roku and phone on the same network?
My Smart TV has a built in app for casting, have you looked through your Roku apps to see if it has an app for casting?
Let us know what you find. Until then best wishes.