Unlocking your router generally voids the manufacturer's warranty. Proceed with caution. Flashing incorrect firmware can "brick" your device (render it unusable). The links provided below point to trusted third-party repositories and forums.
Official and "General" (unbranded) firmware versions for the E5336 series can be found on technical repositories like e5336bs2 unlock firmware link
Once you have located a valid firmware link and downloaded the .zip or .exe file, follow these general steps: Connect your E5336 to the PC via USB. e5336bs2 unlock firmware link