This is an issue of network recognition of Windows. In my computer this problem appeared with both Windows 7 and 10 when I go online via USB modem (Dongle), but not with wifi. But it worked out with USB modem and connected to the Viber, when I connected in some other way without using mobile partner.
Connect to the dongle, through the network connections icon in the right side corner of task bar. Click your service provider name that comes under “Mobile broadbad connection” and click “connect”. Give the APN of your service provider and keep the name and password blank and click next.
Eventually the mobile partner also indicates connected and Viber goes online.