I use a number of older routers just as switches/wireless APs without DHCP. Are you now using all static addresses or does your Mifi provide NAT and allow multiple devices (or maybe you only use it on one machine)?.
If only one computer gets the Internet, you have the option of still using the router and then bridging the adapters to share the Internet connection along with the NAS. You can also use your Linksys as a wireless bridge or repeater to extend your signal, which is particularly easy if you use dd-wrt, open-wrt, or tomato firmware.