DIAGNOSIS The Switch and Steam Deck output video over USB-C, but they can be picky about power handshakes. The "No Signal" means the monitor isn't receiving a compatible video signal from that port.
FIX You must provide external power to the monitor. Connect your Switch/Steam Deck to the monitor's primary USB-C port (for video). Then, connect a USB-C power adapter (like the one that came with the device) to the monitor's second USB-C port (the power-in port). The portable device alone often can't power the display.
IF THAT DIDN'T WORK The issue might be the specific USB-C cable. Not all cables support the specific video protocol these gaming devices use. Try a different, high-quality USB-C cable known to work for video. Also, ensure your Switch is docked or awakened from sleep before connecting.
HEADS UP Even with external power, some portable monitors won't charge your Switch or Steam Deck while in use—they'll just slow the battery drain. Plan for a separate charger for your device if you're gaming for long sessions.