The main cause of signal variation or low speed of a WiFi connection is the use of a saturated or interference channel. Some router models have a built-in spectrum analyzer that checks all the different channels and searches for the least saturated one at the moment (less WiFi signals broadcasting on that channel) and configures a network on a different channel each time.
However, in models that do not have this spectrum analyzer, you have to manually configure the channel by choosing the one with the least network overhead.
There are many applications to perform this type of task, however, the most practical are those available for smartphones since we can place the device just above the router and obtain better results as we would obtain them in a normal way.
A useful application to perform this task is WiFi Analyzer. This application is available for free in the Play Store for Android devices.
The use of WiFi Analyzer is very simple. Once installed on our Android we execute it and we will see a graph with all the WiFi signals that we detect at our reach.
This information is useful, although it can be somewhat complicated to interpret. To obtain more simple information we must slide the screen from right to left and we will change screens. For example, we will see a signal meter that will help us locate the signals.
A list with all available signs.
And, one of the most important functions, a channel analyzer that will show us in the form of stars the most recommended channels for our WiFi depending on how saturated the spectrum is in our location.
As we can see, in our example it recommends us to use channel 1, 2 if not, or 14. Because not all devices support the 5Ghz of broadcast, channel 14 is completely available, but we will lose compatibility, so the most advisable will be channel 1 as we can see.