You try to access your service with Host() && PathPrefix(). But is your target service actually responding to the path /nextcloud? Did you configure it to have that base path?
Usually you want (complex) services to be available on their own domain. PathPrefix always makes trouble for interactive web applications, because they mostly respond with fixed links like /login or /static/js/script.js.
You could use various (paid for) subdomains and use CNAME to your DynDNS name.
PS: recently posted a config for NextCloud