Plates were working but now overexposed.

It is possible that your IR filter is stuck...that would explain not seeing an improvement with the extra IR and the poor images at night in general
I tested the illuminator with 3 different camera models at about 35 feet and it just wasn't very bright. At closer distances under 10 feet it helped. I think it's just under powered or maybe 850nm is not a good wavelength for my cameras or maybe the existing EXIR lights on the camera are brighter than the external illuminator.
 
I'm finally getting good results!:) I switched back to the 16mm bullet (DS-2CD2232-I5) and reverted the firmware to 5.4.0.

Brightness 52
Contrast 75
Saturation 63
Hue 27
Sharpness 50
Exposure Time 1/750
Gain 10
Smart Supplement Light Off
BLC Area Off
WDR Off
Digital Noise Reduction Off
Resolution 1920x1080P
Bitrate Type Variable
Video Quality Highest
Frame Rate 30
Max Bitrate 12288
I Frame Interval 10
SVC Off

I'm getting about 90% success rate now. The failures are do insufficient light. Unfortunately my AliExpress IR illuminator sucks. The plates had better focus with an exposure time of 1/1000 but I missed too many because of dark lighting. 1/750 is a little blurry but still legible. I think if I could get a more powerful illuminator and run at 1/1000 it would be better. I also found lowering the resolution from 3MP to 2MP helped because at 3MP the frame rate is limited to 20. At 2MP I can set the frame rate to 30.

Plates091917.jpg