My Mercury MW4530R wireless router was used as an AP under original firmware, and its 2.4GHz is getting less and worse stable. So I switch back to LEDE. As an open-source third-party firmware, LEDE works fine for a dumb AP on MW4530R except the low speed rate and poor signal - or, in other words, it doesn't stand as a dumb AP.

Enable HT40

Under HT40, the router will bind channels to tranmit in MIMO mode to archive higher rate. Speaking of 802.11n, one channel is 20MHz and provides 150Mbps rate, and MIMO can bind 2 channels to transmit at 300Mbps on 2.4GHz^[1] or 3 channel to transmit at 450 Mbps on 5GHz.
In LEDE/OpenWRT, option htmode is used to specify MIMO mode, with possible values none for NOT using 802.11n, HT20 for single channel, HT40 for auto binding and HT40+ and HT40- for manual specified binding. If 802.11ac is supported, VHT40, VHT80 and VHT160 are also available to choose from. There is another option noscan, which will let the system ignore channel overlapping if set to '1'. And we can modify the configuration file /etc/config/wireless.

config wifi-device 'radio0'
...
    option htmode 'HT40'
    option noscan '1'
...

Save and reload the network, and that's it. But wait! Why does my 2.4GHz network still works at 54Mbps, and 5GHz network can't even start?
According to OpenWRT Ticket System^[2], WMM (multi-queue) is mandatory to be enabled to use HT modes. So there is one more step in the /etc/config/wireless.

config wifi-iface 'default_radio0'
    ...
    option wmm '1'
    ...

Save and restart, then everything works as expected. This is that tiny options which cost my whole night.

Signal and Antenna Design Defects

Mercury MW4530R and TP-Link WDR4300 are sister devices. They shares almost the same hardware, and original firmwares are inter-operative. There are still some significant differences between them, and one of them is the number and configuration antenna. Based on the teardown^[3], WDR4300 features 2 x 2.4GHz antenna and 3 x 5GHz antenna, while MW4530R features 3 dual band antenna. Unfortunately, the antenna is designed for 2.4GHz and 5.8GHz, which works better with CN region bands. The fact that signal is poorly weak when Wi-Fi channel is set to 36 and region code JP confirms that.
Since MW4530R has SMA interface and all the antennas are replaceable, new antennas may save the device from this dilemma.