After investigating the issue with the Shelly Wave i4 and Shelly Wave i4DC devices in SmartThings, we have determined the likely reason why these devices appear offline in the gateway.
The i4 has the Control Scene command class, which is commonly used by remote controls, usually battery-operated devices that enter Sleep Mode. The i4, however, does not have Sleep Mode since it is not a battery-operated device. Despite this, SmartThings "thinks" it is a remote and expects it to enter Sleep Mode, marking it offline (sleeping). Unfortunately, this behaviour is due to SmartThings' programming, and we cannot change it.
Given this situation, there are some options: either leave it as it is, understanding that the device is functioning correctly even if marked offline or use the Red Note or other automation to trigger a refresh of the state periodically every hour or so.