Use 5Ghz band on your router, change channel low band like 40 channel or high 157 channel according what is allowed to your location. Also do you have 24Mbps and up?
Need your router info and ISP info
What do you mean by messes up the wifi?
There are so many possibilities as to what is at fault here - the likliest being your WiFi modem as it appears to be giving bandwidth solely to any device which starts running a bandwidth-heavy app (like Netflix). As someone said earlier, you should also be using the 5GHz rather than 2.4GHz.
Thanks for sharing detailed info. Fix is simple, stop watching Netflix.