How to install DD-WRT on Buffalo WZR-1750DHP Router
you have an older Buffalo WZR-1750DHP router that already supports ac-WLAN, but the firmware is very old? No problem, we just iron a current DD-WRT over it and you have a fantastic router - or access point. It's also a quad Gigabit switch. That means, with five minutes of work you can make the old bargain up to date again! Read the tutorial completely before you start and download all mentioned software. Also important: connect the router to your Mac with an ethernet cable and give your Mac a static IP address.
And off we go: We decide for the free router firmware DD-WRT, because OpenWRT doesn't like Broadcom chipsets too much (no driver access) and the Buffalo is based on Broadcom. Since Buffalo encrypts its own firmware, you can't just install a new one over it, but that's no problem, the clever programmers of DD-WRT have found a way around it (even a very entertaining one). But first of all it's important to find out which original firmware your one comes with: if it's 2.27 or younger it's good, if it's the last official version 2.29, then you'll have to surf to Japan and download the older version 2.27.
Just do a firmware update (actually downgrade) in the official interface under Advanced Settings > Admin > Firmware and then we can do the trick.
The WZR-1750DHP offers an interesting way in the 2.27 firmware how to install other firmware.
So when it runs again or still with 2.27, you should do a 30/30/30 reset first. While the router is plugged in, press the reset button and hold it for 30 seconds. Then switch the router off, but keep the reset button pressed! Do not release it. Again, count to 30. Then switch it back on again, while still holding the reset button another 30 seconds. So you hold the reset button for 90 seconds with the router on, off and on again.
No log on to the router (IP address 192.168.1.1) with the name "bufpy" and the password "otdpopypassword". Sounds strange, but came from the factory.
Now you are logged in and click on the grey tile in the bottom right corner of the user interface which leads to the advanced settings (software version 2.27 is now mentioned under the tiles of course).
On the left side of the menu bar you will find a small button called debug-disp. Click on it.
Another menu opens. Choose Admin Config, then Name.
As well as then at the following three possibilities to the last one, which "firmwup" has in the name.
There we can now add our desired software to the system, namely the special edition of DD-WRT, which you then simply download here. It is named factory-to-dd-wrt.bin - just type in "WZR" in the search field and pick the 1750DHP. This piece of software is extra for flashing DD-WRT on the original firmware. Later updates can be made with the "webflash" variant. The factory-to-dd-wrt you only need this one time.
Now a firmware update window appears again. It's best to do something else for five to ten minutes and don't interrupt the process. 
After a reboot the router should be reachable unter 192.168.1.1
Isn't he? Don't panic. Pull the current and leave it there for now (no kidding, that was the case with our specimen). Also important: do another 30/30/30 reset. Your old hardware is now ready to use with modern software.