When telnetting to port 5000, a disconnection from the telnet session does not allow me to telnet again without hard resetting the entire modem.
Your problem of telnetting to port 5000 is probably caused by the PPP application. If you are not planning on using the PPP portion of the modem / router then go and disable PPP. You can do this by opening a web browser and entering the IP address of the modem/router into the address bar. Once you are connected to the modem/router, log in with the username and password. Once in, click on the "PPP" tab. Under the "PPP General" section, next to "PPP", select "Disable". Then scroll to the bottom, click on "Submit", and then click on "Save&Restart" on the top-right corner.