Some technical forums host firmware dumps extracted from working devices. For example, the Russian repair forum contains a firmware archive for the cpvra1e0804 (mainboard ahb7004t-gs-v3 v3.02 , CPU Hi3520). However:
Insert the USB drive into the USB port of your NVR. cpvra1e0804 firmware download link
Leave the system completely alone during the progress bar animation. Some technical forums host firmware dumps extracted from