You are not alone my friend there are hundreds of us on here with this problem. No solution yet and it has been over six months. There is a reddit post on fortnite and an epic team member acknowledged they are aware of the situation and how the servers are handling udp packets from Cox and its issue it has on the clients upload side. That was some time ago and havent heard anything since. I will look for that post and paste it here.
Just a a COX tech out to my house who rewired the coaxial cable, however the problem persists. He was with me in my office watching the packet loss % stat on Fortnite, consistent 1-5% packet loss. He told me he would talk to his superiors once he's back to the office.