If the default passwords don't work, the ISP likely changed them. You can revert the device to its original state, though this may delete your current internet configuration. Locate the hole on the side or back of the device. Use a pin to press and hold the button for more than 10 seconds while the device is powered on.

The Huawei HG8145v5 is a beast of a device. It is a shame to let it live its life as a dumb terminal. Unlocking it transforms it from their appliance into your gateway.