Out of the box, the Sonoff adapter is setup to be used as a coordinator, like the SkyConnect. To run it as a relay you will need to reflash its firmware. Sonoff has firmware available for that and I’ve seen guides floating around on how to do so.
Hardware side, best I can give you is a solid maybe, and I couldn’t tell you how stable the connection would be. I do know that it has better signal stock than the Nabu’s SkyConnect.
If you stuck it in front of a broad wave guide you might be able to eak out a little more range without limiting its visibility to the sides too badly. Like how satellite dishes work. A tin can would be too focused for this application but a metal colander or a foil lined dinner plate might work. Both the coordinator and the devices you are wanting to connect would have to be within “sight” of the Sonoff’s signal though.
Alternatively, and I don’t know if the parts exist to do this, you could split the single antenna off to 2 antennas, like CB antennas on heavy trucks here in the States. I don’t remotely understand how it works, but if it’s setup correctly, it would increase range to the “front“ and “back” while significantly reducing range to the sides.