It depends on what you mean by "5G". There's a 5g (five gigahertz) wifi band that has had Roku support on some models for some time now. There's also an emerging cellular technology called "5G" (next step up from current 4G cellular), but that's not wifi.
Now, could you receive "5G" cellular on your cell phone and then run a mobile hotspot to your Roku? Sure, but you'll probably be throttled and run out of data.
If you want to know which Roku's support 5g wifi , however, just go here and click through the specs for the higher end models: