I would have setup this router as a WAP by using the LAN port instead of the WAN port.
And be sure you know, I do not offer any support for your current setup. The owner must be able to fix this as it's a little odd to have this many LANs plus the router and PC entries are going to be advanced.
Following is the topology of my network...
My router gets a IP 126.96.36.199 (DHCP) from my ISP by connecting via PPPoE.
Router Gateway is 188.8.131.52
My PC's LAN ip is set via DHCP to 192.168.1.100 and gateway as 192.168.1.1
In this case how do I access my Friend's PC and how can he access mine for file sharing.
Prior to buying a router we were connected directly via switch. Now router poses a problem.