Do you get the error when connected via cable? If so, it is your device.

Check you "guest" settings as you may be able to do it there.

I have had times when http://tplinklogin.net/ does not work for me and must use 192.168.0.1 on my Archer C8. Try powering the router off and on and try again. Also you may need to clear your browser...

Is your router a US model with US firmware? If not, that could be the issue. https://www.electronics-notes.com/articles/connectivity/wifi-ieee-802-11/channels-frequencies-bands-bandwidth.php shows...

I just re-read the thread. If you were outside of your network (on some other WIFI), you would get to your cameras using your ISP IP IPv4 address and the port number. If you ISP IP address is static,...

If it is static you should not need the DDNS. If it should stop working in the future, check the ISP IP address first. Mine has remained static for over a year, however I am using a DDNS as that was...