@mbellinger2 You don’t need to whitelist viber.com unless it’s blocked by OpenDNS. However, since Viber is an application and it uses multiple protocols (including peer-to-peer), you must uncheck the “Block literal IP addresses” and “Block proxy, VPN, TOR” checkboxes in order for it to work. This might compromise the protection provided by pcWRT if your child is tech savvy and knows how to use these techniques to bypass parental control.
If Viber is used on parent’s devices only, you can put these devices in a separate profile and turn off parental control for that profile.