Nextcloud Android app doesn't want to open video

Hi everybody,

this is my first post and I’m very new to Nextcloud.
I have installed Nextcloud on a my PC (eeePC) at my home; I want to use it to store and access my files from everywhere, so I configured all needings (router ports, ecc.); I haven’t a fixed IP, so I use a dyndns to access to eeePC from outside. Nextcloud is installed on a Debian 9.0 system in my case.
I have installed the Android client on my phone, and after creating the user (my user) I uploaded a video, and then I shared it by link.
The problem is : I can access, open and play the video from my PC, as from local IP (192.168…) as from web address (by dyndns service); in both case the server send me the video; I tried to send the link to a my friend, and he could open and play the video; but, when I try to pen the video from my own phone, it couldn’t; all is stopped on a rotating arrow waiting for the video; i repeat, at the same time, the same video, by its sharing link, is visible and playing on the PC.
Any idea?

many thanks