Thank you.
You are right, other than the default firewall rulesets on WAN (eth0), there are no other rules when Basic Setup wizard is configured. What I ended up doing was, to set an IP manually for eth3 interface (e.g. 192.168.3.1/24), then create a new DHCP server (e.g. LAN3, 192.168.3.0) and finally just map the printers MAC address to a static IP (e.g. 192.168.3.2).
I think the issue was that I couldn't get to eth3 gateway, presumably 192.168.3.1, from the printer, even though the IP had been statically set (192.168.3.2).
For windows, once I installed the driver (Brother's website doesn't identify any specific driver for Postscript or PCL for this particular model). Regardless, I recall, when I was setting up the printer with my old router (which only had a single subset), I had to do the followings (Windows 10, but should be very close to Windows 8.1):
- Control Panel -> Devices and Printers
- Right click on an installed printer if you have already installed the driver but having issues (otherwise, look at the note below) and select Printer Properties
- Go to Ports tab
- If you don't see a "Standard TCP/IP Port" port, "Add Port..." and select "Standard TCP/IP Port" and follow the wizard to set one up
- Otherwise, select that "Standard TCP/IP Port" port and click "Configure Port..."
- in Port Settings tab, you can modify your printer's IP (I used LPR protocol and gave the Queue a name)
Note: You can create this "Standard TCP/IP Port" during Brother's driver installation also. Follow the installation in "For Network Printing using TCP/IP (Peer-to-Peer Printing)" section of this page.
I was able to print from both Android and iOS devices (Brother provides an app for each platform including Windows phones too), two laptops on WiFi (Windows 7 and Windows 8.1), and from both my desktops (Windows 10 and Linux).