FTP access something strange

Hello, I have always used an FTP server and could access from the LAN and from WAN.
But since a few days I can only access by LAN and not by WAN.

The strange thing is that if I access from Filezilla or from Windows Network Access does not work and shows the following log:

firewall, info dstnat: in: pppoe-out2 out: (unknown 0), proto TCP (SYN), 90.165.157.208:50367->XX.XX.XX.XX:21, len 52

But I do access tests with https://ftptest.net/ access without problems and shows the log:

firewall, info dstnat: in: pppoe-out2 out: (unknown 0), proto TCP (SYN), 136.243.154.86:40206->XX.XX.XX.XX:21, len 60

The only difference between the log in len 52 and len 60 (what does len mean?)

That could be happening?
Thank you.