thats difficult to do without being able to access the router. that is one of the things you should be sure of before you update the firmware (tama and compatible ang firmware)
also, before you flashed the new firmware did you make a backup of the old one? kung wala, try mo sa site mismo ng prolink baka meron dun
swerte mo kung ang router mo merong flash memory that stores the original firmware so it can just flash back to original firmware but I doubt it. this feature is usually just available on the very high end routers.