Is it possible to configure the router so that on LAN it accepts both FTP and SFTP, while on WAN only SFTP is available? If I check "Enable TLS support" SFTP gets enabled, but FTP is disabled (which actually does not correspond to the button name).
"Enable TLS support" is for FTPS support. SFTP is something completely different and not available on the router unless you install an entware package (openssh-sftp-server).
If you don't have sftp-server installed, you can just use SCP protocol (instead of SFTP), of course, you need to find a client that supports SCP, for example, winscp: https://winscp.net/eng/docs/scp