Now I understand why the DNZ was enabled.
Because without the DNZ that point to dietpi’s IP from outside my network I can’t reach it (using mydomain.ddns.net of course).
ahmm did you set port forwarding on your router? Usually there is no need to create DMZ.
Yes I’ve checked it. Both port (80 and 443) point to the dietpi’s IP.
Should I forward some other port?
80 and 443 should be fine. Nothing else needed. And you are not able to access owncloud (from external) with these 2 ports being forwarded? If yes, ask the provider of your router device how to manage access to your owncloud. This seems to be something the provider should be able to answer.
Hi again,
here I am with some news…
I’ve just spoken with the provider call center and they tell me I have to enter the Owncloud configuration and add the alternative DNS I can see in the router.
I find the alternative DNS (which is a strange number like this 2a:Oe:123:O::etc etc), but I don’t know where to configure this number in the owncloud settings… but I’m not sure they’ve understood which is my problem
I’ve noticed also that the DNS mode in my router is DHCPv6. could this help tom understand better?
Another stupid question: looking at the above information, could it be I’ve to set something different in dietpi, to allow mydomain to reach the server only when I’m in my own network?
this has nothing to do with DietPi. For DietPi is doesn’t matter how you reach the system. At least not, as long as you don’t have any firewall installed blocking access from a specific network. But this should not be the case.
For me the culprit seems to be the router still, and the configuration around. My question would be do you have some kind of DS-lite internet access? I mean, do you have an external IPv4 address?
I probably haven’t read through everything carefully, but since you have HTTPS enabled and probably enforced on the webserver, it makes sense that you cannot access it with your local IP (at least not without actively ignoring client warnings), since the certificate is only valid for the public domain. As fast as you use a public domain and have HTTPS enabled, there shouldn’t be any reason to use the local IP anymore, is it?
there shouldn’t be any reason to use the local IP anymore, is it?
That’s exactly the issue. The web server is not reachable by DDNS from local network. And from external internet web server is reachable only, if the DietPi device will be put into a DMZ on the router.
Does the router allow to add the public DDNS domain as local hostname to the DietPi system (respectively in Pi-hole, if it is the DHCP server), so that it resolves internally to a local IP and does not exit the LAN in the first place? Of course it can be set as hostname on the DietPi system, but the DNS server needs to know about it as well.
I don’t know much about DMZ, sounds like a restricted network then 
.
What does it means exactly?
BTW The call center calls me back in order to understand better the problem. They told me there is no way to modify the router settings to solve the issue.
Honestly, my impression is they are not really prepared and the router can do this job. For this reason, I would try to list the router’s configuration pages, because I think you can be more helpful than them.
The first page is “Connection”.
On this page I can set:
The local IP (IPv4 with the DHCP range and IPv6 with the possibility to enable the DHCPv6 range)
Then I have the “Firewall” page where I can set different security levels for IPv4 and IPv6
Then I have the advanced settings.
On this page I can manage:
The Port forwarding (Actually forwarded 80 and 443)
The Port Triggering (actually disabled)
The remote Access (I guess to the router, and actually disabled)
The DMZ (actually enabled towards dietpi local IP because without this I can’t reach the server from outside the network)
I hope this can illuminate you with some solutions, otherwise, I’ll be back with my old and good pi-hole 
if DMZ is enabled, are you able to reach Ownclound by your local network IP address (not the DDNS). Try http://<local.ip>/owncloud as well as https://<local.ip>/owncloud
And is there an explanation what the DMZ means? and What does it do? I mean in the context of your router. In theory I know what a DMZ means on IT world 
On the “Connection” page where you can set the DHCP range, can you define local hostnames for DHCP clients and reserved IPs, i.e. a fixed IP, managed via DHCP, for clients?
yes, I can reach it but, as you can imagine, is not really comfortable changing the owncloud access every time I go and come back from home.
If I understood well reading the router instructions, it is something that drives the external traffic to a specific IP address (in fact I set the dietpi local IP). The problem is that I can set only one IP here
The only thing I can see similar at what you say is in the connected devices, where there is a list of devices.
Here I can see:
the hostname, if it has a dedicated IP or DHCP, which kind of connection it has (Ethernet or wifi 2,4/5 ghz), and the modify button.
if I go to the modify page of a specific device, I can only choose between DHCP or dedicated IP and which is the IP.
But what if we change the dietpi’s hostname with mydomain.ddns.net?
If the DietPi system is the only host where public connections should go to, then yes it makes sense to change its hostname to match the public DDNS domain. If DietPi has a static IP, you may need to switch to DHCP one time so that the router catches the hostname, at last in my LAN this is needed.
Still not sure why the DMZ is needed to get pubic access, and what it is really needed for. When pubic ports 80 and 443 are forwarded to the DietPi system, it should just work.
hopefully the router is not adding an own domain to the host name like .local or .lan. Some router do that to identify local system.
At the end using PiHole as workaround would be still a good option. It will give more control about the DNS/DHCP stuff, compare to what your router is able to do. As benefit you will get an AdBlocker for your local network. But yeah, you would need to keep it running 24/7, which is usually the aim of Ownlcoud