I did these, I already have WAN usage.
I forwarded the ports.
If I turn off Wi-Fi on the phone, I get to the Nexcloud Clipboard.
But when I am in the LAN, the xxx.duckdns.org link directs me to the modem.
I have the same problem in macom 's tutorial and your tutorial. Of course if this is a problem
Maybe it's a cache issue, where you tried before?
I would try this... open a browser in Privacy/Incognito mode... and go to subdomain.duckdns.org and see if it gives you a secure login. If it does you know it's a cache issue. To fix that, just go to subdomain.duckdns.org (on a regular non private browser) and Hold Control and click F5. That should refresh it to a secure site.