Zic :
So that means I'm wasteing my money and I should just buy the one that comes with the plan
Depends on the MODEM your plan provides, and whether you pay a monthly FEE for that. I paid for my router over time and it's a better one.
A "Gateway" device contains a MODEM and a ROUTER.
MODEM: for connecting to the INTERNET (limited by ISP plan)
ROUTER: for connecting your computers, media boxes etc together AND for connecting them to the internet through the MODEM
*So the ROUTER matters for connecting local devices together. They use a WIRED (Ethernet) and WIRELESS (Wi-Fi) approach:
a) Ethernet - look for "Gigabit" as the 10/100 is limited to about 11MB/second.
(I have a WD MYCLOUD device which is attached via Ethernet. It's a 4TB HDD unit and I can read and write at 90MBps but it would be 8x slower if the router was only 10/100... probably most or all are Gigabit now but just check).
b) Wi-Fi - a bit confusing, but "AC" is the best though you need a Wi-Fi adapter that is also "AC" or it reverts to the lower version. I would look for one that has DUAL BAND of 2.4GHz and 5GHz even if you don't see an immediate need. We'll see more 5GHz devices in the future, and sometimes 2.4GHz devices can interfere (like wireless phones when in use).
*So the speed of WIRED and WIRELESS devices locally (not through the network as that's bottlenecked by your ISP) is limited by the ROUTER.
Frankly, most people would see no benefit to a better router than what most ISP providers give you now so suit yourself. If it's Gigabit, and dual-band, AND the monthly fee is pretty low I wouldn't bother.
**The larger the house (or if obstructions) the more likely it is you need a better Wi-Fi router. Both the Wi-Fi router AND the Wi-Fi adapter the device has affect transmission.
You can always buy a router LATER and send the ISP router back.