What make and models are the routers?
If both routers have LAN ports you may be able to do something like this:
ISP > ---------RJ45/ethernet cable --------> [WAN port] old Wifi router [LAN port] >------- RJ45/ethernet cable --------> [LAN port] new Wifi router.
Knowing more about the routers will help but add some explanation regarding the desired connections between the devices.
Wireless may be a option as well.
The devices will all need to be appropriately configured to make your network work.