If the microwave is "leaking" enough to knock out the roku WiFi signal, I would suggest that you buy a new microwave as they are not supposed to leak that much!
Also, put the TV/Roku on a surge suppressing power strip, that might help. Long shot: buy a clip on RF choke from an electronic supply place and route the roku cable through that (round cylinder thing). Should not be expensive.
The older Roku Premiere/Premiere+ (4620/4630) are dual band capable (2.4ghz and 5ghz). The newer Roku Premiere/Premiere+ (3920/3921) are only 2.4ghz capable (b/g/n).
Since microwaves apparently operate at about 2.45 ghz, it is often suggested to try manually assigning your wireless channel within your router to Channel 1 (2.412 ghz) or Channel 6 (2.437 ghz) in order to "avoid" the interference caused by the microwave, rather than set to "Auto", if relocating either the microwave, the router, or the device (Roku) is impractical.
You may have to try a few channels in between 1 and 6, but if channel 1 works for you, I would keep it there. Additionally, I would lower the Channel Width/Bandwidth to 20mhz in your router settings as well to help minimize interference/noise.