In the Soviet sci-fi film "Napoleon-gas" (1925) they use radio-controlled flying drones for air warfare. Here is the timed link to this place in the film. I wonder if this is a pure sci-fi element or they already used something like this in 1925?
It's sci-fi. The electronics technology required to miniaturise a radio control system (for a model that size) simply didn't exist. If you look at the size of the "control" unit the men are using, you get some idea of the scale of radio equipment of the time. The first practical radio controls for model aircraft would only appear once transistor technology was invented and even then the 'control' available to the pilot was very limited compared to the RC models of today.
Looking at the models, these appear to be 18 inches or so across. There's no obvious attachment between the propeller and (what I assume to be) the motor, slung underneath. There's what appears to be a radio antennae on the top but nothing which might be a radio. There are no moveable control surfaces, so there's no way of controlling the flight even if a radio control system was on the plane.