Nethserver handles the routing perfectly but the router does not know about the VPN.
It’s like
roadwarrior - www - router - nethserver - vpn network
If there’s no static route the router drops packets to/from VPN.
If the Nethserver would be gateway then you don’t need the static route as Nethserver does the routing.
On the other hand, if the main office OpenVPN server is NOT also the gateway, then whatever machine or router which IS the gateway must know to route 10.3.0.0 subnet 255.255.255.0 to the machine which is running OpenVPN.