FTP access to NAS using IPv6

[AlistairR]

A simple enough task, but I’m missing something somewhere. I’m new to ipv6, so do please state the obvious, as it may not be obvious to me

Setup: Fibre modem -> Eero router -> TP Link AC 1750 router -> Synology DS418

Requirement: FTP access to a share on the NAS from an external device. ISP uses CGNAT, so port forwarding is disabled, hence the need for an ipv6 solution.

FTP is set up on the NAS; using the ipv4 address, the share is accessible using FileZilla & WinSCP from an internal network desktop.

Eero router: IPv6 On
AC 1750: IPv6 on. Internet connection: Dynamic DHPC ( SLAAC / DHCPv6 ) LAN: SLACC & RDNSS

Have taken IPv6 address from the NAS network page and using Ping IPv6 Test - Simultaneously Ping IPv6 From Different Global Locations | KeyCDN Tools I seem to get successful pings from all locations shown.

BUT trying to connect with say FileZilla, using the SLAAC ipv6 address in [ ], I get the following response:

Error: Could not connect to server
Status: Disconnected from server
Status: Connecting to [1234:etc]:21...
Status: Connection attempt failed with "ENETUNREACH - Network unreachable".

What have I missed?
Thanks in advance!