check for trees

Trees could very well be the issue causing your internet speeds to be slow or even unusable.
To check this go outside and look at the device that was installed on the outside of your house. 
Then look where it is pointing, if there are any trees in the way that will cause your internet to slow down. 
The device needs line-of-sight, meaning the device needs to visually see the Access Point distributing internet.
Look below for images with examples of line-of-sight, and no line-of-sight.