Even with a USB power supply, the Shelly H & T is not constantly online. It wakes up when the change in temperature or humidity exceeds the set threshold, or periodically when a configurable time period has elapsed. You can also wake up the Shelly manually by pressing the button (small hole at the bottom of the unit through which you can insert a bent paper clip, for example).

If you want the Shelly to wake up as often as possible:
- Set the threshold value for the temperature to 1 °C.
- Set the humidity threshold to 0.5%.
- Activate "External Power Supply" (as described by David).

Especially the threshold value for the humidity is exceeded every 5 minutes due to slight fluctuations. And the setting for the external power supply also causes me to wake up every 10 minutes (not every hour like David).

Have you also set this in the sensor settings?
Sensor Settings -> External Power Supply -> Activate
The sensor then transmits every hour instead of only when the threshold values are exceeded or undershot.