A "300 mbit" router might get you 60Mbit WIFI. To get 100Mbit WAN service, you have to have a router with gigabit wired ports. To get 100Mbit WIFI you probably have to have a 5Ghz WIFI. Your current router has neither. If you have no money, but want an improvement, then go to E-Bay and buy a used Asus RT-N56U, RT-N65U. If you have a little more money you could get an RT-AC68U.
The "300" part of the name is just marketing. It is the maximum link rate that you could POSSIBLY get. In the real world you won't get it.