So, your Wi-Fi network choice is only one name, correct? In that case, it is a simultaneous dual band wireless network.....meaning that the same wireless network name is used for both the slower 2.4 GHz signals and faster 5 GHz signals.
In that case, it is up to the Wi-Fi device to select the "best" signal.....either 2.4 GHz or 5 GHz.....with which to connect. The big question here is what is "best"?
If "best" means the strongest signal, then the phone will connect at the slower 2.4 GHz rate....because those signals are stronger than 5 GHz signals. Top speed on most 2.4 GHz networks is about 130 Mbps. I'm guessing that is how your phone is connecting right now.
If "best" means the fastest signal, then the phone connects to the higher speed 5 GHz band.
Given a choice, most Wi-Fi devices will connect to the strongest signal.....not the fastest signal.
If this is what is happening on your network, the only way to make the phone connect to the faster signal might be is to set up the Wi-Fi router to provide separate 2.4 GHz and 5 GHz networks with different names and "point" the phone at the 5 GHz network. So, the 2.4 GHz network has one wireless network name and the faster 5 GHz network has a different name. Then, it's easy to select the network that you want to use.
But....there are some downsides to using 5 GHz.....so things are not quite as easy as it might appear.
So, before we throw in the towel on your current network, let's see if we can get the phone to connect to the faster 5 GHz band.
Power off the phone and wait 10-15 seconds
Move the phone to the same room as your Wi-Fi router
Power up the phone and let it connect
What kind of speed do you see when you run a few speed checks?