ok there's nothing wrong with power levels in router and SNR is fine too, but not at the box, this points to a duff cable from splitter to the box.
if you have a different one, swap this over from splitter to the box, if you have no spare cable, take the one from the router and put this in zgemma, and the one from zgemma into router, but before that, check that you have the connections the correct way at the splitter, the side with one inlet should be connected to incoming VM cable, the two out ports should be connected to box and router, I've seen people get these wrong before