That the open port checker would only show it open if there was some app that responded on port 3700. Since I only have what's in your post I can't make other guesses but openport checker may fail.
In fact your site/link given failed on ports I know are open. So it's a crappy tester.
I have got tp-link TD-W8961NB router. I tried to open specific port (3700) by port forwarding on my local ip (192.168.1.3) but without result. Adding rule to windows firewall or turning it off doesn´t help. I also tried to turn off antivirus and router firewall but without result. Port is still closed. I tried to check port status with open port checker and ping port paping.exe PUBLIC_IP -p PORT -c 4. Port 443 is also closed and i can´t open it.