This time developers had designed a solution were a WebJob (Azure App Service) would be created in Azure, but it would connect to on-premises network to the database.
In this case Site-to-Site nework for VNET was created with static routing because of limitation in on-premises firewall.
When we tried to add the WebJob (Azure App Service) to this VNET we received an error that it can be added only to networks with dynamic routing.
As explained here https://azure.microsoft.com/en-us/documentation/articles/web-sites-integrate-with-vnet/ that is the limitation of Azure App Services, because it connects to network with small VPN module and that technology depends on dynamic routing.
The easiest solutions here are to use Azure App Service Environemt (which is a Premium feature and thus costs more) or Hybrid Connections (https://azure.microsoft.com/en-us/documentation/articles/integration-hybrid-connection-overview/)