Yeah, pics of the amp settings, wiring, subs / wiring may help.
I don’t know the box setup or if the subs were taken out / rewired, but make sure all the coils are in phase properly if they were out of the box since last used.
Using the voltmeter, play a sinewave (60hz roughly), then turn up the volume and amp gain to see how many volts AC the amp is putting out at the speaker terminals (make sure the crossover is set to let the test sinewave pass), with the volume at zero should be 0 volts, and cranked up should be over 60vAC (but depends what that amps true output is, so I suspect it might be a little less). Unplug the RCA cables and measure AC voltage at the ends of the cables, test left and right separately when playing the sinewave, do it with volume at zero, then turn it up to max and should get roughly what the head unit preout voltage is rated at or close. Then you know the amp is at least getting full input signal.
I’ve run plenty of cheap stuff so not saying anything bad about it, but that amp isn’t going to be the best quality and I would not be surprised if it developed an issue. But, start with the wiring, make sure the subs are all good, RCA cables, lastly, cycle all the amps switches and knobs (dirty switches and knobs can kill or hinder output). If you have another amp to install / test, that might be the quickest thing to try.