I have the same camera. I was also trying to download the latest firmware before installing, but was told by tech support the camera has the latest firmware installed already. The firmware version on my camera is 188.8.131.52_18051102. I downloaded RLC-420_1389_18081400.zip.
From tech support:
It seems you used the new version camera and the firmware is for the old version camera.
The RLC-420-5MP released later than RLC-420. So the firmware updated for RLC-420 is not applied to for RLC-420-5MP.