You usually only use one router on a home network unless you want an isolated subnet. So I'm going to guess that you either want to turn off the DHCP in the N300, or more likely want to use the Thompson as a wireless access point.
To keep the N300 as your modem/router and use the Thompson as an AP, just attach it to the N300 with an LAN to LAN connection, turn off DHCP in the Thompson, assign it a static address on both the Thompson and N300 with an address that is in the N300 network range but not in the DHCP assignment range. Then use the same wireless security type/encryption/and password as the N300 but a different radio channel to prevent interference.