All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
Our advice to you is to inform your neighbor of installing the internet cable passing through his roof; if he approves of it, then there is no problem and you bear no sin for it. If he does not approve, then it is more prudent that you move this cable away from his roof, regardless of whether its presence harms him or not.
Al-ʻUmraani wrote:
"Chapter on digging a waterway or sewer in the neighbor's land or roof: It is impermissible to make a water passage in someone's land or on his roof without his permission. This is the later view of Ash-Shaafiʻi. In his earlier view, he said, 'If a man digs a spring or well and he is required to pay its costs and necessity entails that he makes it pass in another person's land and this does not cause significant harm to that other person, then some of our scholars (Shaafiʻis) held that the other person should be forced to accept it.'" [Al-Bayaan fi Mathhab Al-Imaam Ash-Shaafiʻi, 6/258]
Allah knows best.