It depends what you mean "not very good". Other routers may have extra features you want or maybe different WiFi. It will not be much different if you connect via a ethernet cable since that is a very simple function and ever garbage routers do ok at that.\
The device you buy must have a modem in it if your are going to completely replace the current device. A router that only has a ethernet WAN port only connects to a modem of some type...most times a cable modem but it can connect to any. You might use your current router/modem as just a modem and then buy a router with a ethernet wan. It again goes back to why your current router is "not very good"