A couple things.
1. The signal is technically within optimium range. The problem is your signal should be within roughly 3.5dbmv of each other. yours ranges from 3.9 to -3.9. dbmv. A 7.8dbmv range.
2. For testing speed, directly connect to the modem. Its pointless to troubleshoot the modem if the router is whats causing the slow down.
If you have any splitters, amplifiers, attenuators in your coax line, remove them. check the line for kinks, damage, loose/cheap connectors, or even just cheap cable. I usually tell people to connect the modem right where the coax enters the home. This prevents run within the home from being the issue.
