Hello,
I had a couple of "TL-WR840N<W> V4" to integrate my Home Assistant devices and always saw that they barely passed through 2 walls and already dropped connection with some devices, measuring -94dB and such.
Today I gave my trusty old TL-WR741ND V1 with just one antenna a last go. Same place, same power supply, same channel.
To my surprise it reached all devices AT ONCE and all of them had reasonable signal strength and stopped dropping.
This leads me to believe that the MediaTek driver for OpenWRT is flawed. The 741ND uses Atheros.
Yep, all of them, as it seems. Even my cellphone shows mediocre signal strength when only one wall apart.
This happens with all of the TL-WR840N V4 that I have.
I TFTP'd the same OpenWRT on a new V4 router and somehow, I literally can't explain this, the issue seems to have self-solved itself.
My initial idea was to perform a performance test with both stock and OpenWRT, but... yea.
Remember the "Generic 802.11bgn" appearing on the MediaTek on my first post? Well, now it's properly written "MediaTek MT76x8 802.11bgn" there.
It goes much MUCH farther now and all devices connect with similar signal strengths as the Atheros. No drops.
Also, the 501mW and 1000mW options are gone, in all countries. Now it "only" goes up to "22db (158mW)", but as I said, it goes MUCH further.
I'm not sure what happened there, but I guess the /var/config/wireless file was somehow messed up, so it used a generic driver, and a fresh flash fixed it? Or even the driver itself? But why? This makes no sense.