Put modem WAN into Bridge/Bridge mode. On its LAN side the Westell 7500 has a DHCP address server. To retain access to its settings, you need to keep the its LAN active so that the modem has an IP address (for example 192.168.1.1) and turn off the modem's DHCP adress server. Disabling the modem's DHCP server keeps it from intercepting your router's request for an IP address and allows your router to get its Internet IP address from CenturyLink. Unless you plan to access the modem via its radio, you also need to disable its WiFi radio.
Your router's LAN addresses need to be on a separate subnet from the modem (for example, 192.168.x.y where x is not 1).