A. Win10 PC, OpenVPN client 1 (192.168.3.101/24)
B. OpenVPN server (192.168.3.12/24)
C. RUT240, OpenVPN client 2 (192.168.3.105/24)
D. LAN PC(?) (192.168.5.10/24)
Do you have a static route leading from C -> D? How are the two networks connected? (192.168.5.1/24 and 192.168.3.1/24)
Have you correctly set up your client-config-dir? See: '–push <option>' & '–ifconfig-push <local> <remote-netmask>' @ (https://openvpn.net/community-resources/reference-manual-for-openvpn-2-4/) What is the device running the OpenVPN server?
If you cannot change the IP subnets for the devices, a better alternative would be to use 'TAP' instead of 'TUN'. It should fix most of your problems without the need to configure a 'client-config-dir'. Then there's only the matter of routing from C -> D.