So schedule an SSH connection to call /export on them, as my backup system does.
some routers are behind NAT
So port-forward SSH out to where it can be reached, or use a VPN, or set all of them up on ZeroTier, or…
This forum is filled with solutions to that problem.
no FTP
It’s easily enabled on every server-class OS.
It’s not particularly secure, but if you do it over a VPN, it doesn’t matter.
Also, some FTP servers have a “dropbox” mode where files can be added but not downloaded again, which can be helpful in cases like this.
nor SSH/SCP in my case.
Also easily added, especially the client side as in my backup script.