Question Weird Throttle Behavior Over WiFi

Jun 29, 2020
I have a very weird problem. My wifi connection is behaving like it's throttled by ISP.

Speedtests (near locations only), Netflix, Youtube, Steam etc, are fast (around 27Mbps) but most of the other things are around 15-20 Mbps.
I assumed my ISP throttling my connection which makes sense because when they claim they are better than other ISPs they talk about Netflix etc popular sites.

BUT the weird thing is when I tried to connect with ethernet cable, suddenly I have consistent 27Mbps everywhere. (Also the lag is gone while gaming) No throttle what so ever.

I get the same max download speeds with both but throttle only aplies on wifi. What? I don't understand.

Things I tried:
Tried different pc's and mobile phones even: Same behavior.
I disconnected every other device and put the connecting device dead near to router: Same behavior.
I glued a 12cm fan on router and now it's very cool: Same behavior.
I fiddled a bit with the settings on the router interface: Same behavior.

I downloaded a file that gets throttled while watching 4K Youtube video: Success! Combined speed is 27Mbps. QoS problem? Why affects wireless connection only?

ISP: TurkNet
DSL Modulation Type:VDSL2
Main Laptop: Asus X550JK
Router: TP-LINK TD-W9970 (Someother ISP's custom version. Can't update firmware to 2019 one and if I downgrade it to 2016, I can't even revert it back to 2017 one so I didn't tried to downgrade, last resort)

EDIT: I solved it! Two ways to solve this. First way is disabling 802.11n and using 802.11g. It worked but was slower than 802.11n so wasn't worth it.

Second way is disabling WMM. When using 802.11n, my router won't let me disable WMM so I disabled it through my wireless adapter's driver settings on Windows and it worked.

When using 802.11g, enabling or disabling WMM doesn't change anything that I can notice but 802.11n on my side is a nightmare with WMM enabled.
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