You may need a larger attenuator @piotr337. I think the noise floor for the radio is quite low.
Another thing you can do without buying another attenuator is place the boards in something that will naturally attenuate your signal. Metal box, etc.
Another pro tip is to use the AT+CFUN=0 command to force disconnect your device, see how it handles it and then AT+CFUN=1 later on to make sure it connects again, etc. This definitely helped me work out some logic around disconnects/spotty connections.