TL-W810N with a custom firmware

Hello there,
Recently I've buyed a TP-LINK TL-WR810N(EU) v1.1 router, just to make a network storage on my home.
Unfortunatly the router came with a custom firmware, something called RouteBilling (links to prints bellow):
Login Page:
Another pages:
I've tryed to install OpenWrt via TFTP, but nothing happens when I choose the directory from renamed file.
The proccess to recognize the router happen, but when at the time to chose the directory, nothing.
Some one can help me, please?
Thanks in advance!