The site is inaccessible on any other device that is on the same network.
I have set up port forwarding on my router (External port: 20-20,
Internal port: 21-21, protocol: TCP), however, the warning:
"Note: Access to the modem FTP Server via FTP port change to 2121 on
the WAN. On the LAN is still on port 21." appears.
Port forwarding on the router will only affect external access. You need to change the local firewall settings on the laptop.
However, if Windows Firewall is completely turned off, it is accessible.
See below for instructions.
How to Configure Windows Firewall for a Passive Mode FTP Server
Configure the FTP service to only use a limited number of ports for passive mode FTP
In the IIS 7.0 Manager, in the Connections pane, click the top node for your server.
In the details pane, double-click FTP Firewall Support.
Enter the range of port numbers that you want the FTP service to use. For example, 41000-41099 allows the server to support 100 passive
mode data connections simultaneously.
Enter the external IPv4 address of the firewall through which the data connections arrive.
In the Actions pane, click Apply to save your settings.
You must also create a firewall rule on the FTP server to allow
inbound connections on the ports you configured in the previous
procedure. Although you could create a rule that specifies the ports
by number, it is easier to create a rule that opens any port on which
the FTP service is listening. You limit the ports on which FTP is
listening by following the steps in the previous procedure.
Configure an inbound firewall rule to allow inbound FTP connections to only the ports on which FTP is listening
Open an Administrator command-prompt. Click Start, click All Programs, click Accessories, right-click Command Prompt, and then
click Run as Administrator.
Run the following command:
netsh advfirewall firewall add rule name=”FTP Service” action=allow service=ftpsvc protocol=TCP dir=in
Finally, disable stateful FTP filtering so that the firewall does not block any FTP traffic.
netsh advfirewall set global StatefulFTP disable
Source How to Configure Windows Firewall for a Passive Mode FTP Server