I used to have good wifi anywhere in the house. Now i have only wifi when i sit next to the wifi monitor

Take a look at this Apple Support article:

https://support.apple.com/en-us/HT204051

Reset your network settings.

Settings - General - Reset - Reset Network Settings

Reset your device. This won't delete your data stored on the phone.

• Press and hold the Sleep/Wake button
• Press and hold on the Home button
• Keep holding both buttons until the display turns off and back on with Apple logo on it.

Alternatively, you may go to Settings - General - Reset - Reset All Settings

Additionally provide more information about your environment. If you live in an apartment building it is possible somebody next door set up their wi-fi on the same channel and is blocking your use. It is even possible you have have done this to yourself if you have multiple routers or users.