Buy a network extender or invest in a mesh network.

There is a ton of variables to getting fast speed out of wifi, signal strength, interference, your ISP download/upload speed, network congestion, to name a few, and we can’t figure those out for you over the internet.

it’s likely a dual band router that has 2.4ghz wireless “g” (for compatibility) and 5ghz at a faster speed.

The 2.4Ghz will likely be limited to 54mbps, and is likely your slowdown.