Do you have a Android based mobile e.g. Samsung etc NOT Apple? Go to the Google playstore and find "WiFi Analyser", then you can walk room to room and measure signal strength, and from neighbours houses as well (Very important!).
Make a note of which Channel numbers (1-13) are in use by your next door neigbours in both 2.4Ghz and 5Ghz (36-140). If your next door uses Channel 1 you need to set on your WiFi Router 4 numbers higher or lower than theirs (if you can). This should fix most signal strength issues.
This is where I would start