Are you sure you have the speedtest figures the right way round? Most internet service has much less up than down
If you are truly only getting 3mbs down, well that's your problem. HD needs between 2-4mbs, and sounds like your router is giving priority to Roku, not computer, so if speed test truly says 3mbs, then there is nothing left.
4k needs 20+, by the way, SD a bit less than 2.
Not a Roku employee