From an existing router, can i connect an ethernet cable to my aiport express to extend its wi-fi signal throughout our home?

I've been trying out different internet service providers for a long time, and i recently tested speeds in different places in my house. Obviously, the nearer i am to the router the faster the speed is. I tried installing cisco signal extenders but it doesnt really prove to be effective in increasing both speed and signal in my house. Would it be useful to place my airport express in the places that have low signal and have an ethernet cable from an exisiting wi-fi router connected to my airport express to extend its signal? Or is it still more effective to have a cable from the modem itself?