Been getting denial of service attacks recently need help

Recently family a member has been streaming his video game to a streaming website and a person who none of us knows has been shutting down not only the internet but our phone service as well. we have pretty good internet and were curious if there was a way to monitor the incoming traffic to figure out where it is coming from. The family member doesn't interact with this person(s) at all and like clockwork, every time he connects to there website to stream what he is playing the interruption to the service starts about 30 mins. to an hour into his streaming until he has to stop streaming. I did some digging around and found out the person(s) involved have been violating 18 U.S.C. section 1030 of the Computer Fraud and Harm Act of 1988. but the larger issue at hand is the phone which gets cut off when this person(s) does this action putting a family member that has medical issues in harm's way because the individual(s) are shutting down the modem and our phone services. When the family member who streams tried to call to get help the service rep was trying to sell him unwanted products and refused to help them out. please any help would be appreciated.