you will have to isolate your old router, and change the IP range it uses.... the subnet should be the same, and yes, you should turn off your new routers DHCP.... (do this by turning off Wireless on your computer, and connect to the router with the LAN cable, and make sure it's not connected to the other one, then you can access the page(s))
next you would have to set it to the correct mode, as plugging it into one of the LAN ports, won't provide wireless access, because the router it's self needs an IP address, and it won't get one of these via a LAN port... it would have to be a 'cable' modem and into the WAN port to accomplish what you wanted, unless your router supports bridge mode.
trying to use a router as a wireless access point / extender is hit and miss usually as some will work fine and support what's needed, and some wont
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