Why is a LAN client machine unreacheable when the client is connected to OpenVPN?

Question

I am trying to understand why the following is a problem.

1. I have an ASUS RT68U Router on which i have enabled the OpenVPN Server.
2. I can successfully connect to the OpenVPN Server using an external connection e.g. use OpenVPN app on iOS to connect with .opvn exported file. Once i am connected to the OpenVPN Server, I can successfully connect to local clients on the LAN (e.g. Mac Pro, the Router).
3. However, if i turn on OpenVPN Client connection on the Mac Pro using TunnelBlick e.g. connect to PIA on the Mac Pro, the Mac Pro will become unreachable via OpenVPN Server connection (e.g. cannot ping the machine anymore). As soon as the Mac Pro disconnects from PIA, it is reachable again.

Why is this? Is there some kind of conflict occurring. I have spent two hours trying to understand the problem.

Apologies if the description is hazy, here are the various configurations that work and do not work where the second configuration is the one i am trying to understand:

1. External Client ---> OpenVPN Server ---> Router ---> Mac Pro : REACHABLE

2. External Client ---> OpenVPN Server ---> Router ---> OpenVPN Client ---> Mac Pro : NOT REACHABLE

3. Internal Client ---> Router ---> OpenVPN Client ---> Mac Pro : REACHABLE

Answer 1

Okay solved this once i got a better understanding of how VPN works (thanks to the OpenVPN forum)!

In short, when i was connecting to the PIA VPN client on the Mac Pro the Mac would get a virtual subnet and IP (10.x.x.x) as opposed to the IP address allocated via the routers DHCP and subnet (192.168.x.x etc).

So i had to put a route in on the Mac Pro which told the Mac Pro about the virtual subnet of the OPENVPN SERVER and the gateway for the ROUTER. The following command allowed me to do this:

sudo route add -host 10.8.0.0 -netmask 255.255.255.0 192.168.1.1

Where 10.8.0.0 is the OPENVPN SERVER subnet and 192.168.1.1 is the ROUTER GATEWAY. The Mac Pro was immediately reachable.