Network (broadband) speed and WiFi range are not linked, especially if the range had dropped off a cliff *.
I’m guessing someone else’s WiFi has changed to use the same channel as yours.
Quick fix is to reboot the router and it should pick a less congested channel.
If that doesn’t fix it then get a WiFi scanner app for your phone and see what shows up. Make sure you check 2.4ghz and 5ghz. Then login to the router and change its channel to a less congested one.
Also are you connecting to the 2.4ghz or 5ghz network. 2.4ghz doesn’t offer as much bandwidth (speed) but does have a greater range, generally.
* WiFi speed will slow down as the distance from the router increases by that’s different to your broadband speed.