- The first thing I notice is that you wrote about https, but acording to the linked site, the web(cam)server on the pi supports http, only. Maybe, you give it a try?
- Test if you can connect to the server from within your local network, i.e. from your mobile when connected to WiFi by visiting http://raspberrypi:8081 (Maybe, you need to use the IP address instead of the name of the PI.)
- Cross-check your IP. Visit for example https://www.whatismyip.com from within your local network to find out your IPv4 address, and check out if this IP is also currently stored at your DDNS provider.
- What you wrote about your router config seems to be correct, but without knowing the router and what exactly you configured there, it's hard to tell you something.
Finally, there could be a problem if you use IPv6. With IPv6, every device on earth can have its own IP, and port forwarding should be obsolete. This implies that the PI itself updates the DDNS service with its own IPv6, and not of the IPv6 of the router. And the router must be configured to open the firewall to let requests on port 8081 from outside in to the PI. Unfortunately, configuring IPv6 can be a nightmare or even impossible on many routers.
And... A IPv4 only client can not connect to a IPv6 only server.
Some of my friends and me meet regularly after work at one of us, an one friend runs a webserver on his PI with a calendar about when and where to meet next. He changed to a cable ISP, which only assigns IPv6 addresses here. Unfortunately, most workplaces and all except one mobile ISP assign IPv4, only. So, we can visit the server only from home, but not from the mobile, nor from the workplace :-(