Wi-fi internet stop working when I connect my macbook air

hello, I am Gio from italy.

I'm in a trouble with internet and macbook air: when I connect my macbook air to the wi-fi network, internet become very slow or stop working on every conncected devices (also in macbook air).

what should I do?

the problem begins when the logic board of the macbook was change.

Macbook air is a middle-2011, 13'', 1.8 Ghz, i7, 4gb, 250gb, yosemite.

WI-fi router is airport; iphones, macbook pro, another macbook air are working fine.

thank you in advance.