Configure firewall rules at Site B

From the Firewall menu, choose Rules. Open the WAN tab, unless using a different interface for the VPN connection. Click on the + button to add a new rule.

Enter these values:

ActionPass
Disablednot checked
InterfaceWAN
TCP/IP VersionIPv4
ProtocolUDP
Sourceany
DestinationType: WAN address
Destination port rangefrom: (other) 9876 to: (other)
Lognot checked
DescriptionSite-to-site VPN

Click Save and on the next page click Apply changes.

Click on the OpenVPN tab. We’ll now add a rule to allow traffic through the OpenVPN connection. Click on the + button add a rule.

Enter these values:

ActionPass 
Disablednot checked 
InterfaceOpenVPN 
TCP/IP VersionIPv4 
Protocolany 
Sourceany 
Destinationany 
Lognot checked 
DescriptionAllow everything through OpenVPN 

Click Save and on the next page Apply Changes.