First, make sure that DHT is checked in Options > Task BitTorrent. Also rightr click on the download and check in 'Advanced' as occasionally it doesn't set DHT.
Also try a 'high' port number after setting a fixed LAN IP number in the computer > network settings. Open the router firmware and set that port to the IP number and port matching the one you just set.
Try a complete reboot first: power down BOTH computer and modem/router, wait at least two minutes, restart the modem/router and, when it 'locks', restart the computer.
One other possibility is the use of a VPN which