Unable to connect add *cloud from Thunderbird

Greetings,
I run NCP on a pi. No docker.
NCP if behin VPN. No direct access to port 80 and 443

In front of NCP I have an other Pi, outside the VPN scope. It is exposed.
In this Pi I also run the duckdns updated in a docker and Nginx PM (of course, both exposed)
I am able to access my Nextcloud from outside using the domain name: name.duckdns.org
I am UNABLE to access my next cloud using the same public (sub)domain name. But I am able to access it using : https://nextcloudpi and https://192.168.x.x

I am using Thunderbird as a mail client in my laptop.
Before adding Nginx, I could use the addon: ADD *cloud so that all attachments above 5MB would be saved in the cloud and send a link to the recipient.

Since I added Nginx PM, I am unable to configure the same plugin. No matter what I use; official name or local IP.
I cant figure out what is going on.
Any help is appreciate.
Thank you

UPDATE
When not using my network (so from an outside connection), and setting nextcloud in the Addcloud* configuration, it works well.
Therefore I can confirm the cloud do work.
The situation is the interaction with Nginx that I cannot resolve.
Help needed… thank you

You should set up split-horizon DNS on your LAN so the public name resolves to the LAN IP while on you LAN.

Thanks
From what I read about split-horizon DNS, is a whole installation of a new dedicated DNS on my network. Is that right?
At the moment I use openwrt with openvpn client installed for the all network. The dns point to my pihole that goes to my VPN provider.
Do you think I can make that working?
thanks

I resolved the issue to HOW to connect to my nextcloud within my network when in a VPN umbrella.
I excluded the NCP address from the scope of the VPN.
Now when typing the sub.duckdns.org address, I can access the cloud.

Yet, something is not working when trying to access it via the thunderbird.

I understand that this might be on the edge of NCP related… but eventually is connected as I have no idea if I have to modify some parameters in NCP to have all other element working.
As it was for the article in doc nextcloud
https://docs.nextcloud.com/server/19/admin_manual/configuration_server/reverse_proxy_configuration.html
I hope to find help between this community and the thunderbird community.

Thank you for suggesting more options.
Kind regards