Amplifier WIFI signal a Box

For our clients who ask us, almost all, to have a WiFi connection, we had a problem connecting to WiFi Broadband in a small remote guest house only fifty meters from our main house which is located a "Nine-Box"; amplifying the WIFI signal transmitted by two blocks or CPL LEA 200S SFR200 (we tried both) was really small with an ordinary router Net-Gear Style 108 Mbps. Using the Asus RT-N66U router has enabled us to triple the signal strength. We dare think that would be the same in the immediate vicinity of the small guest house in the garden but the waves are struggling to pass the stone walls of the fourteenth century. However, it is sufficient on a 5m radius of the cottage.