5 Ghz, not 5G. Two different things... 
What model Roku device do you have? Not all devices have dual band wireless radios, so they only support 2.4 GHz.
Of the currently available models in the US, only the Ultra and Streaming Stick+ have dual band radios. The Express and Premiere models are single band, and you have to go several generations back to find other models with dual band radios.
Dan
Roku Community Streaming Expert
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