@indefinity 

Sadly this is too common an issue, basically 2.4ghz is a flooded spectrum and will cause you issues, see it all the time.   Its just something you have to work with, move all you can to 5ghz.

Quick fixes..

On an android phone get WiFi Analyser, scan the 2.4 and see what frequencies are free (likely none)..  choose the best (one with lowest signals) and set router for it.   Remember that only 1  6 and 11 dont interfere, so choosing   2   3 4 or 5 will interfere with 1 and 6..   7  8  9  and 10 will interfere with 6 and 11..  you get the idea!

If all is flooded, try 12 or 13 (if they are options)  .. its on its own and can fix a lot of issues.. however it wont work with all devices.  Some older devices are only channels 1-11

Change mixed mode to N Only.. again older devices wont work, but will help speeds!  

Keep on 20mhz if congestion is high, 40mhz only make it worse!

See how you get on..  

Sadly, not a lot you can do about this as 2.4ghz is a flooded spectrum for 95% of people.   I live in the country and back on to a cow farm, still get random interference on 2.4ghz!!