When you have one of those routers like the Planex (AKA PCi) MZK-W04NU, the first thing to do if to upgrade the firmware to an ‘open’ version, so it does what you want it to do, not what the manufacturer had in mind.
In the case of the above router it has WiFi 802.11n, a 4 port switch and one more NIC and one USB port. That’s enough to make it do a lot of things: router, wireless router, or bridge, or firewall, or media server, or NFS/CIFS server, or print server. You are limited by the CPU (MIPS 4k@400MHz) and RAM (32 MByte).
Step one is to update the firmware to something else, e.g. OpenWRT.
In the end you will have one way or another a “webui” file which you can use to update the firmware via the firmware update link in the web interface.
Once you have OpenWRT installed, it’s easier: you need a “bin” file (about 7.5MB in size), and upload it via:
mtd -r write openwrt-ar71xx-mzk-w04nu-squashfs.bin firmware
Then wait (it takes about 3min), it will reboot, and there you go: a new firmware.