My ISP has given a modem & a router. I can only connect to the modem with the Mac id of ISP's router. I mostly use for gaming n video streaming. So i wanna know what is the best way to connect out of the following?

1- modem -> deco as router (mac cloning with old router) - > deco as router

Or

2- modem - > old router - > deco as router - > deco as router

Or 

3- modem - > old router - > deco as AP - > deco as AP 

What is the best one and what is the advantage??