I don't know anything about sigfox, but I know a fair bit about radios.
If operating from other locations which are a similar distance from the peer sigfox device is successful, then there is a good chance that, all other things being equal, you have a local noise source that is decreasing the signal to noise ratio in your sigfox receiver below that at which the modem works.
Options to mitigate this might include some combination of:
Operating from a different location
Using a directional antenna pointed at the peer node. This may increase received signal and reduce pickup of local noise.
Identify and remove the noise source, or if very localised, move your device a bit further away.
Operate on a different frequency or an entirely different frequency band.
You could try using some sort of communications receiver or scanner to listen to the operating frequency, but these days using something like an RTL-SDR with suitable software defined radio software should allow you to see the state of the appropriate part of the radio spectrum and ascertain what kind of problem you may have.
As a radio ham, with lots of radios, I find that they have a high tendency to interfere with each other, even when operating in separate bands, as they are usually harmonically related, thus transmissions around 145 MHZ interfered with stuff around 435 MHz, whilst stuff on 434MHz hit stuff in the 868MHz ISM band. Note that USB cables have a tendency to radiate strongly around the 430-440MHz band and may affect the 868MHz band quite strongly too, so you might have some luck in avoiding proximity to USB cables and perhaps also monitors or TV screens.
