Yes, whatismyip.com return booth IPv4 and IPv6. The Windows and Unix commands also worked. IPv6 support on my router is also enabled. IPv6 (Native) support.
That’s not a RFC1918 address, so most likely you don’t have a double NAT situation. It’s just a question of whether your ISP is blocking your port(s). This is what the tcpdump test can determine.
If you run tcpdump on ports 80 and 443, and you see packets coming to you from the internet (not counting from your LAN), then that means the port is unblocked and your router setup is working. Note that you don’t necessarily want Nextcloud served on port 80, but you need that port open if you plan to use certbot.
Based on what you’ve said, I think the tcpdump filter you need to use is ’tcp dst port 443 or 80 and not src net 192.168.0.0/16’. Look for packets coming in for both ports. You can try to connect to it from cellular or something to generate some traffic. If this works then there shouldn’t be any network-related reason for your Nextcloud to not work.
So configure IPv6 port forwarding then 
When you try to access your NC server from the Internet you need to pick the servers IPv6 address global scope.
To test the accessibility of your server, use the server’s IPv6 address in your client’s browser!
For more precise instructions which IPv6 to configure in the browser and which to use for external access, I would need the output of ip addr from your NC server. And while you probably don’t want to share this kind of information publicly I wrote you a private message yesterday which you can reply to.
Ok, i wil run tcpdump and try to connect usining my cellphone. But i have plans to use Let´s Encrypt SSL certificate, but insructions request the port 80 to be open in order to obtain the certificate. UPDATE: No success when trying to connect using my cellphone (4G), tcp dump is listenning or port 80 and 443, communucation only from LAN.
I will try to access the NC server using the IPv6 address then. And send you the output of ip addr by a PM.
If you use certbot’s Apache plugin or equivalent, your webserver must be able to receive on port 80. This does not mean that you have to allow connections to the Nextcloud site on port 80, however. You could serve a blank page or 403 error on port 80 or simply redirect to HTTPS.
If that’s the case, then your ISP is probably blocking those ports, and there isn’t much you can do about that as far as IPv4. Many of them disallow hosting things on residential services as part of their AUP.
Yes, maybe my ISP is blocking the ports. But also, it seems that i will only bem able to host my server using IPv6. But my router doesent have IPv6 port forwarding options, only IPV4. And i need to enable port forwarding for IPv6. My equipment has support for IPv6 for internet connection, but not advanced settings since It recieved the IPv6 support a few months ago , by a system update. It is a limited system. It doesent have all features yet. So it looks like that my problem is deeper than just a connection issue. I will probably need to buy another router and setup everything again. 
But thank you all for your time and assistance. I am really grateful.
There is no port forwarding in IPv6 because there is no NAT in IPv6. Each device has its own public IPv6 address. You just have to make sure your firewall is set to allow the port (and hopefully block other ports).
This is pretty much what he meant. There is just no option to configure the IPv6 firewall and therefore open the required ports for IPv6.
Exactly, at this time there is no way to configure the IPv6 firewall and open the required ports. The port forwarding in my system works like this: http://www.cfos.de/pt-br/cfos-personal-net/port-forwarding/xiaomi-miwifi-mini.htm There is no IPv6 option.
Well as I said, there is no port forwarding for IPv6 because there is no such thing as NAT in IPv6. Port forwarding is a function of NAT.
Is this router even running a firewall on IPv6? It may not be.
I dont think so. There is nothing about a IPv6 firewall in settings or in the user manual.
Did you put your FQDN line in Trusted Domains section in config.php?
A bit off topic but in my experience the easiest way to setup access outside the LAN is to run the Nextcloud instance as a hidden service. Now it might sound difficult to get tor up and running, but it was not. In running NC on Centos7 so I just installed the tor package from the Epel repo and uncommented two lines from the default configuration and that was it. I could then access my NC instance from anywhere in the world with the Tor browser. Now many clients don’t support connecting to a hidden service so that might be a limiting factor, so YMMV
I never think about trying this, it is a different way. Thanks for the tip