@ionutm 

The port forwarding looks good, but I would change a couple of things or try.

Change the Deco WAN connection to DHCP, and test the remote FTP connection. This is to make sure all WAN settings on the Deco are correct, not a permeant solution, but to test.

Remove the Deco from your network, connect your HP computer directly to the modem, reconfigure the port forwarding and test. If that fails then the modem is blocking FTP traffic.

Change the port forwarding in the Deco from the service of FTP to Custom.

Add another port forwarding rule for both routers for port 20 in addition to port 21.