External.access can be enabled from Settings - Access section.
The easiest way to enable external access is to set Automatic IP detection and Automatic port mapping and save.
If UPnP is enabled on your router than you should get port mappings created automatically for you.
If you get default ports created (80 and 443) you are good otherwise you may need to use manual port mapping mode.
Manual port mapping
In case of UPnP is not available on the router or it creates non default ports you will need to create port mappings manually on your router.
Follow documentation for your router. Usually it is very intuitive just open your routers web page.
You need to create two port mappings:
Router port 80 -> Device port 80, TCP
Router port 443 -> Device port 443, TCP
Many Routers use Application form under Firewall settings. You should create application (Syncloud) with the above two port mappings and then assign it to your Syncloud device. After that you should be able to enable external access.
Automatic IP detection allows DNS server to use source IP address of the DNS update request.
In rare cases it is not your public IP then you will need to specify it manually.
Non default ports
Default certificate validation port is 80. When it is not available on your router device will not be able to get real certificate from Let's Encrypt service. That is their requirement. Browser will not be able to validate your server validity.
Default access port is 443 (HTTPS). When this port is not available on your router you may not be able to access your device from some networks with strict firewall rules.
Public IP (IPv4) on device
If you have a public IP you do not need to enable external access as your device is already visible from the Internet.
IPv6 on device
If you have IPv6 it is public by definition and you do not need to enable external access.
You will be able to access your devcie from IPv6 networks only.
To be able to access your device from any network (until everyone migrates to IPv6) you need to ask your provider to give you public IPv4 address (for your router) and enable IPv4 mode for your device (on your router). After that you should be able to enable external access.