Hallo mysensors community!

I am totally new to everything here but slowly making my way through the various posts on how to build sensors properly!

My goal is to build battery-powered window sensors!
My setup: 3,3v Mini Pro 8Mhz, nrf-radios, serial-gateway with arduino nano hooked to a raspberry pi with home assistant running!

My problem:
I´ve build a sensor based on the circuit from @petewill shown here: https://forum.mysensors.org/topic/6734/video-how-to-battery-powered-chair-occupancy-contact-sensor and the example binary-sensor sketch in the arduino mysesnsors library.
When my 3,3v mini pro is still hooked to my pc via usb and ftdi-adapter, it works perfectly fine! I change the switch, the mini pro reacts instantly, sends a signal, the gateway leds green and yellow blink once and the state of the sensor in home-assistant changes within a second.
BUT
when I disconnect my mini pro from my pc and hook it up to the two aa-batteries (like in the circuit above) the whole process just refuses to work.
When I change the switch in battery mode absolutely nothing happens! But sometimes, like every 30 seconds or so, the mini pro just lights up, sends something, and the gateway-leds green and yellow blink multiple times. This is completely different to the usb-powered mode!
Also, the changed state of the switch is not transmitted to home-assistant within this automatic, untriggered, sending-phase!

I don´t think it is a range-issue, since I am still having sensor and gateway right next to each other on my table.
I´ve tried putting a 47 uF capacitor between vcc and gnd of the radio, too.

So what could I do? I definitely need your help here!
Thank you all in advance! Looking forward to your answers!