Serious vpn issues

I’m using a sonic wall firewall on a clients network. After setting up the VPN they way they instructed it didn’t work.
I went thru the configuration with a fine tooth comb and found no discrepancies.
So I carried my laptop(192.168.168.x) to the client connected my laptop to the LAN Modem(192.168.168.x), which is connected to the external side of the firewall(192.168.168.X).
The internal address of the firewall is (192.168.1.X)
Suffice to say, it didn’t work.
After paying for support from the company I was told that what I was doing could NEVER work because the Laptop was on the same subnet as the firewall.
My question is why does that make a difference ?