No matter what the B2C shows, the label on your router shows that it's definitely WZR-HP-G300NH2, so you should use DD-WRT firmware for WZR-HP-G300NH2 only (you can find it on buffalo's offical website). I've flashed a same router with firmware wzrhpg300nh2-pro-v24sp2-19154 downloaded from buffalo site.
You may try this:
1. Set you my IPv4 IP/gateway/subnet, you have already done this;
2. Run cmd, type this command: ping 192.168.11.1 /t;
3. Connect your computer's to the router's LAN port (you should got 4 black LAN port) with router's power unpluged;
4. Open tftp program, set server 192.168.11.1 and locate the firmware file;
5. Focus on the cmd window, and plug in the router' power, when cmd shows you got reply from 192.168.11.1, press Upgrade button on TFTP program.
6. If upgrade successful, just wait until the router finish reboot.