You should get a new microwave. It's probably not the mains circuit, it's probably leaking enough radiation to overload the RF frontend on the router.
If the router gets enough energy the "sine wave" of the radio signal starts to flatten out at the top and bottom and becomes a rounded off square wave which we call "clipping". This has the original frequency component, but also a ton of other frequency components that push up into much higher frequency bands like 5GHz.
Fun fact, this is why electric guitars add a high pitch scratchy noise, they're reaching the distortion/clipping threshold of their amplifier.
If the router gets enough energy the "sine wave" of the radio signal starts to flatten out at the top and bottom and becomes a rounded off square wave which we call "clipping". This has the original frequency component, but also a ton of other frequency components that push up into much higher frequency bands like 5GHz.
Fun fact, this is why electric guitars add a high pitch scratchy noise, they're reaching the distortion/clipping threshold of their amplifier.