Upgrade Router and modem or just get an extender?

I recently got Internet with spectrum for 1000mbps because they were the only company that offered cable to my apartment.

When using the Spectrum speed test it says the router is receiving 1000mgps but my personal device is only receiving 650mbps when next to the router and only 240mbps when 30 feet and on the second floor (The furthest room in my apartment) Yet I still experience long buffering times from both distances

I've been trying to look for a Wi-Fi extender but should i upgrade the router and modem?, The router just by itself?, or just Upgrade the router and get an extender?

And if so if I could get some recommendations because this is all new for me, Thank you.