You can only do it by connecting them by a (long) ethernet cable or power-line plugs.
What are the IP addresses of the two routers, as in 192.168.0.1/192.168.1.1/192.168.1.254 and so on? You probably need to change that of the old one to something suitable, which we can tell you once we know them both. Then you disable the DHCP server on the old one and connect them up.
Will you need a laptop or other mobile device to keep working when moving around, or do you just want better wireless coverage? I think there is a way of doing the first, as someone claimed to have done it, but just at the moment I don't remember the detail
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