@skragamuffin, the DLNA media server function from the OS3 that was powered by Twonky Media is now an app by itself in the OS 5. You can install that app from the App Store to stream to your smart TV. You can refer to this article:
https://support-en.wd.com/app/answers/detail/a_id/29630/kw/os%205
And if you’ve already installed it but having detection issue, this other article may help:
https://support-en.wd.com/app/answers/detail/a_id/29985/kw/os%205
