Further to the other suggestions, and definitely after having replaced the batteries with new lithium, try setting the IR illuminator to “Always on”, just for a test. This will fault find if it is the IR illuminator itself, or the day night sensor section of the circuit, and will be useful fault finding information for when you contact support. If the illuminator comes on you should be able to see it as a red LED. Some report it as purple, but it depends on your sensitivity to it. Do it with the camera in a darkened room, if it’s easy to bring inside, to speed things along.
Then again, not that it matters, because if it still doesn’t work after new batteries, then it’s faulty, either way and would need to be replaced or repaired. From my perspective though, I would have to know, and for sure, if it was out of warranty, and was the IR illuminator only, I would replace that.