If you are connected to Wi-Fi and you are able to browse the web, but still cannot establish a Viber call, the most probable cause is that the required ports are closed.
The ports which are used by Viber are:
TCP: 5242 and 4244
UDP: 5243 and 9785
You should enable those ports (ingoing and outgoing) on your router and your firewall. If it is not your private network, you should contact the network administrator.