Question How to fix 2.4G Slowdown?

Apr 23, 2020
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Sorry for the noob question, but I've already tried everything (at least from what I know) to fix the slower speed on 2.4Ghz. Here's my setup:

ISP's Wifi router (ZTE F660) on the 1st floor at the corner (yeah, thanks ISP for outright rejecting my request to have it installed somewhere better), 35Mbps connection
Another Router (Tenda AC7) on the 2nd floor in universal repeater + AP mode, with 5ghz and "dual frequency optimization" enabled

With the ZTE alone, 2.4Ghz on the first floor works fine. Max speed. On the 2nd floor, speed drops to ~20Mbps or even lower, even though the signal is high. Not max, but high. Like 1 bar of wifi just missing probably due to the metal sheet flooring foundation for the 2nd floor. That's why I added another router to become a repeater.

Here's the thing. 5Ghz works like a charm on the Tenda AC7 router. Max speed, no drops. However, if connected to the 2.4Ghz, same thing. Speed drops to like ~20Mbps. I don't think the 2.4Ghz network is congested here since there's only a few APs visible. Tried using inSSIDer, and channel utilization across the board is just 0% or really low.

Things I've done:
  • Changed ZTE settings - tried 20Mhz on CH11, tried other channels as well, same thing. Currently set to auto.
  • Changed Tenda - Universal repeater + ap mode with 5ghz enabled. Tried disabling the dual frequency optimization so I have 2 APs, same thing. Max on 5ghz, speed drops on 2.4ghz
  • Tried placing the Tenda router from different points on the 2nd floor, same thing. Max on 5ghz, drops on 2.4ghz
  • Tried using my spare 2.4ghz router which worked fine on another home, same. Speed drops. The router only supports 2.4ghz.
  • Tried placing the Tenda AC7 beside the ZTE router and connecting it via a Lan cable, using the Tenda AC7 as the main wifi router, and disabling Wifi on the ZTE one. This setup is worse. 5ghz on the 2nd floor when connected is good, but drops connection. 2.4ghz, same issue.
Anyone knows what's wrong with the 2.4Ghz band? Lan cables across 2 floors is out of the question.
 
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It is obvious that there is literally 'something in the air' that is causing problems with 2.4Ghz signals. I bet almost anything using that frequency would have problems, not just wifi. Unfortunately, until you find the source of this signal interference and eliminate it, there's not going to be any solution for you.
 
Yes, obviously. Not sure what though and why the situation I'm in is happening. If there's something that interferes with 2.4ghz, shouldn't the first floor be affected too? I mean, I'm in a pretty small house.
 
Yes, obviously. Not sure what though and why the situation I'm in is happening. If there's something that interferes with 2.4ghz, shouldn't the first floor be affected too? I mean, I'm in a pretty small house.
It depends on where it is coming from. If the signal is from overhead or there's enough around the house on the first floor blocking the signal--who knows.

I mean you can put aluminum foil on the inside of all your exterior walls and it will probably stop it, but then you might get some weird looks, lol.