I would say you have a serious grounding issue, or a loose power connection on the mains. I can't believe any electrical noise would fowl up a plugin router, that is pretty much not possible. I could see a plug in router or a VFD creating electrical noise, but not normally the other way around.
Maybe a field strength meter would work, but not sure about that. https://www.ebay.com/b/rf-field-stre.../bn_7024797861 A cheap transistor radio might work also, I didn't know they still made these. https://www.amazon.com/Battery-Opera.../dp/B076DK947N Just tune between stations, the buzz should get louder as you approach the source.