VegasRooted
If you select and HIGHLIGHT Settings/Audio/Digital output format = Passthrough, what formats are detected? (The box on the right will state the detected formats).
If you don't see at least DD+ (Dolby Digital+) listed, you wont be getting 5.1 (or Atmos) for almost every streaming service/app (The Roku Channel, P+, and Apple TV aside).
However, unfortunately OS14 (and most RokuOS versions/builds) has Sound Modes enabled by default (including on a new 4850), and these disable multichannel output, so you need to disable them:
During playback press Options (*)/Sound Settings/Customize sound modes/Volume Leveling & Dialog Enhancement = Off
NOTE: a Factory Reset will return RokuOS to its default configuration including Sound Modes being enabled by default, so you will need to disable them again afterwards.