Debugging unstable ports from Cable Modem + Router

Question

Question: are there logging tools that I can host locally/remotely to check whether certain ports are open 24/7? I am trying to inspect and debug the particular port-forwarding setting on an Asus Cable Modem.

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In my apartment, I have opened two ports on my cable modem + router (Asus CM_32-AC2600) and they turn out to be unstable. My service provider is Xfinity (Comcast). Both ports are reserved for one desktop machine, which I host 24/7. Here goes the more specific usage:

• Through port 33XX, I access the desktop machine through RDP connection. After configuring the relevant Windows Firewall settings, I can establish the connection most of the times. Yet, the problem is:

  • The connection is unstable where the graphics usually freeze for 10-20 seconds. During the time, except for the mouse, not a single pixel on the RDP session will move;
  • At times, I will be prompted with the following warning message
  • In the worst case, there can be more than 10 "Connection attempts", which all would fail;

• Through port 22YY, I access an OpenSSH server hosted on WSL on the desktop. With appropriate firewall configuration, I can SSH into the machine by a simple command: ssh ip_of_the_machine -p 22YY.

  • When the RDP connection breaks, similarly, the SSH connection will be broken as well. New requests will be timed out.

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I narrowed down the symptom to the "Port-setting" based on the following observation:

• When the RDP connection failed, I tested other Wifi-Connected home appliances and they were all online ==> I can turn on/off my AC unit through its app, and I can toggle the Wemo switches. This rules out the possibility that the cable modem (router) is completely offline;
• On the other hand, the desktop machine is connected to the router through Ethernet cable. When I go home to check out the RDP failure, I can still visit the 192.198.1.1 page for router configuration. If the router has internet for Wifi, and that the desktop can keep talking to the router through Ethernet cable, I think I can safely rule out the possibility that my desktop PC should have "intermediate internet connection".
• These bring things down to the Port Forwarding settings on the router.

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More logs of what has been done by Asus: the initial problem hit in December 2018 and they sent me a new modem. During the time of waiting for the replacement modem, I rented a modem + router from Xfinity. Ever since the compliance to the service provider, there had not been a single RDP disconnection problem.