Roku devices cannot be "cast" to without a network connection. They aren't capable of a peer to peer connection. You have to have a WiFi network with both devices connected to the same network for screen mirroring to work.
You ask a question and then declare there's no assistance within a matter of hours? Yes, Roku has poor tech support. But what you're asking is more to do with how to screen mirror your phone, which isn't really a tech question for Roku to answer. At least give it a day for a response. And while it's possible to screen mirror onto a Roku, that's not what they were designed for. It's simply an add-on feature beyond their designed usage.
That's completely wrong and backwards (and contrary to Miracast spec. regardless).
Roku devices use WiFi Direct (WiFi P2P) for Miracast connections, which is required as a base minimum by Miracast spec. Infrastructure (AP) mode is optional and only supported for R2 Miracast devices (which Roku's are not).
Roku devices also use WiFi Direct for other functionality such as the "point anywhere" remote and Hotel & Dorm connect.
Roku's name for WiFi Direct is "Device Connect" (Settings/System/Advanced system settings/Device connect).
If you disable Device Connect (WiFi Direct), you won't be able to screen mirror to the Roku device (because Roku's are R1 devices that only support WiFi Direct/P2P connection mode).
Similarly, if you arent connected to any WiFi AP SSID on your sending device, you will still be able to establish and maintain a screen mirroring connection to a Roku (again, WiFi Direct/P2P).
You can also check/test Roku's WiFi Direct/P2P Miracast connection basis by simply pulling the power from your WiFi modem/router/gateway after establishing a Miracast session to a Roku - it will stay active. If this were a Miracast Infrastructure (AP) Mode connection, it would drop immediately of course.
Lastly, screen mirroring is in fact "what they were designed for" (along with every other bit of functionality), and have been, for many years now: Miracast screen mirroring was added nearly 7 years ago, and is an integral function "out of the box".