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All I can think of is to set the "Bandwidth Management" settings to assign a better priority to a particular type of traffic to the desired WAN interface, besides that I see nothing else to try. Please refer to the article below for details;
Article ID:5748 Use TheGreenBow VPN Client to Connect with RV34x Series
Router Introduction A Virtual Private Network (VPN) connection allows
users to access, send, and receive data to and from a private network by
means of going through a public or share...
Article ID:5728 Configure a Teleworker VPN Client on the RV34x Series
Router Objective The Teleworker VPN Client feature minimizes the
configuration requirements at remote locations by allowing the device to
work as a Cisco VPN hardware client. When the T...
Article ID:5708 Configure the LAN and DHCP Settings on the RV34x Series
Router Objective A Local Area Network (LAN) is a network limited to an
area such as a home or small business that is used to interconnect
devices. LAN settings can be configured to li...