Livebox and Synology NAS

Hello everyone,

I have a Synology NAS and a Livebox 4 at home.

I would like to access this NAS from outside via FTP to upload/retrieve documents. I activated the QuickConnect service on Synology, which doesn't work as well as before but it does the job. It has a static IP set from the box and not in the NAS network settings.

We just moved, switched from Free to Orange, and I don't understand what's happening:
With Free, I had to enable FTP/FTPS and make two settings in the configuration panel of my NAS to access it from the outside with the following address: my.old.public.ip:myport

With high hopes, I wanted to set up the same thing with our new operator, but I can't access my NAS. I searched for an explanation on other forums: sometimes it said we couldn't replicate our old configuration because the Livebox changes public IP every restart, sometimes it said I needed to change ports from 21 to 30000, I don't remember why -> in short, it's a mess.

I turned to no-ip as advised, but even today, I am unable to connect to my NAS via FTP, even though I followed Orange's tutorial to the letter. Moreover, www.whatsmydns.net correctly finds my no-ip host.

I should mention that all my FTP access tests are done using a mobile hotspot.

I'm not at home, so I don't remember the NAT/PAT settings on my box.
Does anyone know what's going wrong? What details do you need?

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