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 (both ingoing and outgoing) on your router and your firewall. If it is not your private network, you will need to contact the network administrator.