If your connection is like that: ISP-----(telephone line)--- TD-W9970---(ethernet cable)---Sonicwall and the ISP provides internet with static IP, not PPPOE or PPPOA(which needs an username and password), this method may work.
1. Set up the TD-W9970 to get internet via static IP
2. Uncheck 'Enable NAT' under Network-->WAN settings-->Advanced in the modem's admin page
3. Connect the Sonicwall and another device which need the other 2 IPs to the TD-W9970 (some other settings may be required from ISP and the Sonicwall manufacturer)
4. Connect another router to the TD-W9970 for wifi if needed (the wifi from TD-W9970 won't work in this mode)