Speed drops to 0.06 mbps when 1 or 2 more devices connect

i am having TL-WR841ND v7 with openwrt-18.06 branch (git-20.319.49209-ab22243) & i use openwrt only through Luci (webpanel) not from shell. My laptop is connected through ethernet and wifi on mobile device. the speed i get is mostly 10mbps which is my connection but when 1 or 2 more devices connect i mean total of 5-6 devices, then my speed drops to 0.06 mbps.... wow! how can i fix this issue? i tried this speed test on ethernet on laptop and wifi on phone both give same results..

Sounds like you have a single device connecting at 802.11g speeds almost?

You can see in the wireless tab which clients have negotiated which speeds up/down with the AP. I'd keep an eye on this as you connect or disconnect all your devices one by one (leave a minute or two between each to be sure the data is up to date).

logread -f in the terminal while you do that might help as well. That doesn't have the lag LuCI has (although LuCI is pretty responsive I'm not sure about the actual refresh rate, might be every few seconds).

i don't really understand what you want me to do can you explain a bit more please? im a newbie to openwrt.
also, the issue is that when total of 6-7 or more devices are connected to my internet speed drops ot 0.06Mbps on all devices even if it is ethernet or wifi, new or old phone/laptop.
should i uncheck "Allow legacy 802.11b rates"?
also force 40Mhz?

Uncheck allow 11b but do not force 40 MHz. Don't set 40 MHz at all unless you have NO neighbors with wifi. It is a very rare situation where 40 MHz on 2.4 is actually an improvement over 20.

Go to the realtime graph of traffic on the WAN interface to see if it's simply some device taking up all your ISP bandwidth. If that is the case SQM will greatly improve performance.

this is the wireless statics of devices, vvcap 2020-12-27-15-53-02
and this is nearby wifi signal statics.

"Mian House" is my home wifi
At last there is no option of sqm or qos.vvcap 2020-12-27-16-00-26
after disabling the allow 11b option, i can feel wireless slower.. but speed tests are ok. i mean browsing speed is slower now, but in speed test its giving good, while enable this option speed and browsing both are ok.

Have you looked at the traffic graphs?

The RF situation looks good-- the co-channel neighbors are all weak and the clients all have good signals (If "Mian brothers" is something you control, set it on channel 6 instead of 5) . All of your clients are n capable so there is no reason to allow b rates.

SQM is a package that needs to be installed separately (luci-app-sqm) then you will have a new option for it on the top bar. The problem is that the WR841 has very little space to install packages, so that may or may not work. Since it is a 4/32 device you are limited to the old 18.06 versions but do update to the latest one 18.06.9.

and yes it is 18.06.9