The user experience in a particular location (can vary by the foot!) is depended on the signal strength and the interference of other radio signals. Also while you might receive a decent signal your relatively weak iPhone transmitter will have trouble sending data back to the nearest WiFi access point causing pages to load slowly and sending large files to fail.
Survey the house using an app like WiFi explorer to see how strong the signal is and if it’s on an as clear as possible channel. You might have to go into the WiFi access points (router) and adjust the channels for the best possible scenario.
Make sure you measure neat the WiFi access point (router) and where you will be using the WiFi. You need to find a clear frequency for both sides…
20/40/80/160 MHz The higher the number the faster the connection. However wider the wider options also receive more interference. A specially in the 2.4Ghz hand where is can cover 2 channels at once. You might want to limit the speed to the lower options. You better have a steady connection than an intermittent fast one…
Repeaters / Mesh network devices:
They must receive a strong signal before they can rebroadcast it.
Daisy changing Wifi repeaters:
Mesh networks benefit from links between multiple devices like a spider web.
If you place mesh or extenders in a daisy chain you will cut the speed in half per link!
You mentioned that you cannot run an ethernet. If there is TV Coax available in the various parts of the house you might want to use a MOCA device to create a stable wired connection from where you can set up a new WiFi access point to prevent the scenario mentioned above.
Avoid roaming issues by using a centralized (mesh) wifi system like Eero so you create a uniform WiFi network. Where you can “jump floors” or avoid daisy chaining by adding a wired connection to these harder to reach location.
Only after you have verified a solid well working LAN (WiFi) network you can start looking at upgrading the ISP to a faster speed. Otherwise only the devices that are in line of sight with the router or are connected via ethernet will benefit from the additional speed.
Some routers manage the speed to the connected devices depending on their need or preferences set in the admin panel. This will keep you from seeing the maximum speed because the router might limit you to keep a reserve for other users…