Um, okay? Unfortunately, that doesn't tell us anything about your network setup.
The bottom line is that Let's Encrypt needs to contact your server ("your server" being whatever you want to serve mdmtopnet.duckdns.org with), on port 80, from around the world. You're responsible to configure your network appropriately to make that happen. I'm going to speculate that, if the Internet connection to your server is via your mobile phone, it isn't possible--but you may need to ask your mobile phone carrier about that.