If you are temporarily using your dad's router as a switch you have to make sure you are not using the WAN port to connect to your router. Remember the "router" (they are not really routers) is doing NAT bridging so the WAN ports and network ports will not pass traffic like a switch. You will need to disable the DHCP and set the local port to something other than 192.168.1.1, just like blogz101 said.
It might be easier to just set your dad's router to defaults and plug the WAN port into your router. Your parent's PCs will be NATing twice to get to the Internet but that should work. (I have done 4 NATs once) You will not have to change any settings when their connection gets set up, just plug the router in and go.
Oh, and ipconfig will save you the reboots.
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