Hello, In what country do you live? I can do it for you for a good price if you'd like. Or if you want to do it for yourself : you NEED a hot air station to replace the WiFi IC, replacing the antenna only wouldn't change anything.
You can at first try another battery, if that doesn't work check the battery connector (especially the middle 2 pins). When it is at the charger, does it drop percentage or does it just not change? You can also try charging it when powered off for 1 hour and then check again.