Wireless problem

I have a longstanding wireless connectivity problem which occurs usually around 19:00-21:00
I have no idea how and why this is happening. Wifi becomes unstable and incredibly slow.
I'm suspecting my brother's LG webOS smart tv

That TV's link speed is way higher than other devices.

config wifi-device 'radio1'
        option type 'mac80211'
        option path '1e140000.pcie/pci0000:00/0000:00:02.0/0000:03:00.0'
        option band '2g'
        option channel 'auto'
        option cell_density '0'
        option htmode 'HT20'

How is that TV able to get 130-144mbps link speed while htmode is HT20?

I have no specific insights but if you suspect a particular device is causing the problem, you can test that by waiting for the usual time you experience the problem and not using that device.

What device? What version of OW are you using? What is the CPU load when that device is pulling the bandwidth?

How fast is the data plan you have from your ISP? Is 130 Mbit/s maxing that out?

Do you have a wired device you can use to test internet responsiveness and speed while the device is drawing that bandwidth? At least isolate the problem to one of the wifi and not the connection itself.

Always set your country in the radio config for best wifi results.

The first and second devices apparently only have one RF chain which limits them to MCS 7 at 72 Mbps. The TV has two RF chains so it can use MIMO to realize double the speed at MCS 15.

Actual throughput to a device is always less than half of the raw radio MCS speed.

Signal levels look good. Do some scans to see what channels the neighbors are using and set your channel to one that doesn't have any strong neighbors. The auto setting is supposed to do that but it may not always work.

Also test ISP speed during peak times. Like darksky asked, what type of Internet connection do you have and what is the rated speed?

@darksky @mk24 I'm watching video from samba share on the LAN.
When wifi becoming unstable, even copying the video file fails saying "network error"

root@OpenWrt:~# cat /etc/os-release
ID_LIKE="lede openwrt"
OPENWRT_RELEASE="OpenWrt SNAPSHOT r21549-963c77158c"
root@OpenWrt:~# lspci
00:00.0 PCI bridge: Device 0e8d:0801 (rev 01)
00:01.0 PCI bridge: Device 0e8d:0801 (rev 01)
00:02.0 PCI bridge: Device 0e8d:0801 (rev 01)
01:00.0 SATA controller: ASMedia Technology Inc. ASM1062 Serial ATA Controller (rev 01)
02:00.0 Network controller: MEDIATEK Corp. MT7612E 802.11acbgn PCI Express Wireless Network Adapter
03:00.0 Network controller: MEDIATEK Corp. Device 7602