Can a router, modem, NAS, NVR or other non-rack device sit on a rack shelf?

Yes. That’s one of the main reasons rack shelves exist.

A shelf lets ordinary non-rack equipment live inside the cabinet without requiring rack-mounting ears of its own. Routers, modems, small NAS units, CCTV recorders and similar devices are common examples.

Check the device’s weight, height, depth and ventilation requirements, and remember that its connectors and cables need somewhere to go as well.

A shelf can turn almost any suitably sized box into a rack resident. It can’t, unfortunately, persuade a large tower server that it has suddenly become 1U.