You can run the small programm âddclientâ on your NextcloudPi which periodically checks if your IP address has changed and in that case updates your FreeDNS entry. It is in the Debian package repository, so you can install it directly. Here is its documentation:
âŚand it explicitly supports FreeDNS.
Ah, cool, I didnât know NCP could do that directly - great tool! 
Hi âgaryfâ, I have the same setup as yourself but with a different router. Trying to keep things simple, I can access my NextCloud RPi2 server from anywhere.
I use a service like no-ip and have my routerâs Dynamic DNS option point to that.
Then, still in my router, the NAT menu option for âVirtual Serversâ is set for the outside port 445 pointing to inside port 443 of my RPi2âs IP.
Also, if you canât find a Dynamic DNS option in your router, you can use the ddclient which was mentioned by âsimonspaâ in a previous posting 
I know different makes of routers use different names for services, but all-in-all theyâre pretty much the same. Hope that helps.
PS - once you get all working, I recommend you start using a https service such Cloudfare, this keeps you safe!
This is solved already
There is depending on your ISP. With a lot of ISPâs you can request to have a static IP address. Some ISPâs charge an extra $5 per month for this, it all depends on the ISP.
No need, we already set up freednsâs dynamic dns service
once you get all working, I recommend you start using a https service such Cloudfare, this keeps you safe!
Cloudflare is not a âhttps serviceâ its a ddos protection, but it tends to be unstable due to unqualified behaviour of their dev, so actually you protect your cloud from failing by using an unstable service, which seems kind of counterproductive.
What should be used is letsencrypt certificates and force https on your server, that is what keeps you safeâŚ