can I change to other modem ?
If you have FTTN/FTTB then you need a modem/router.
For all other NBN types you just need a router.
If you want to use the NBN phone you will still need the 5366, but that can be connected to the other router, it does not have to be the main router.
21? Do you really want to expose that to the internet?
Anyway, basic FTP can have some issues because port 21 is used to do the negotiating of another different port for FTP which will get broken via NAT, so the way to use it is to use Passive FTP I believe (but really you should be using sftp and/or ssh with port mapping on ssh and/or a VPN tunnelling server and VPN client and not exposing very insecure port 21 to the internet!).
This explains the problem you will face with FTP server behind NAT-