Wifi routers operate on microwave frequencies. It's possible that the harmful effects on the seeds were culinary rather than carcinogenic; that is, the seeds' internal temperatures were raised slightly, cooking them to death, instead of genetic damage. On the other hand, a human body has a giant active cooling system (the bloodstream and skin,) so minute temperature variations are less harmful.
Alternate explanation: Based on my understanding of botany, I believe plant seeds usually consist of relatively few unusually large cells. This means there are fewer copies of each chromosome to go around, so damage to one chromosome is much more catastrophic than it would be in an adult human body, where mutations happen all the time and it's really no big deal.
Finally, consider the inverse square law. The amount of radiation, say, two inches from a router, is vastly less than the amount of radiation a foot and a half away.