Newly installed Openwrt and strange 21 OPEN port

Hi, I am totally new to OpenWRT (don't kill me if I am doing something stupid).

I bought a Linksys router, have read OpenWRT documentation and setup my VLANs thanks to great tutorials on https://www.youtube.com/watch?v=qeuZqRqH-ug. My ISP modem is on bridge mode, everything seemed looking well (have connectivity and DNS working, zones blocking correctly, etc).

Now I tried to do some port scanning with NMAP to my public IP from a computer outside of my network. I did some simple TCP scan to all ports (-sT) and found some filtered ports and just port 21 OPEN. I see also run -sV to check this port 21 is tcpwrapped.

I tried then to identify what was listening to port 21 running netstat -nutlp | grep 21 and there is nothing! Then I checked that I have no port forward rules, NAT rules or custom rules, and also no traffic rules mentioning port 21.

I would like to understand how this port is open... any hint?

Could be by your ISP filtering port 21 out.
Try to forwarding it to a FTP server in your LAN and see if you can connect from public IP to it.

I tried connecting via ftp but I get:
ftp: connect: Connection refused

should I use wireshark or another tool to check if this port is reaching the router?

You can use tcpdump on the router CLI, I think the command would be:
tcpdump -i wan port 21
As someone else said, it is probably the ISP blocking them, and the router won't see anything.

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