Outdoor antenna works indoor only

Not continously variable, but stepped like this:
3 dB (attenuator #1)
6 dB (attenuator #2)
9 dB (attenuator #1 + #2)

https://discussions.flightaware.com/t/announcing-the-pro-stick-plus/19202/125

It showed that at my location, the RF interference is so bad that without an external filter, Pro Stick Plus’s front-end LNA gets saturated, and performance drops to near zero.

However if attenuaters are added, these dampen the interfering signals and performance shoots up even without an external filter.