Setting country code to Japan and using channel 14 is the best bet i've got.
I think you will find 5ghz usable with a current mwlwifi. Unfortunately a current one has not been back-ported to stable as of yet.
Speed test 5Ghz with Huawei P10 and Samsung S4, WRT1900acs v.2 with LEDE/Superwrt!
If you want to try SuperWRT, is an alternative firmware for compatible wireless router equipment. The files are provided free of charge on the site and compiled from the public sources of the LEDE project with adjustments in the kernel and system configurations.
The files for installing and updating are found in directories organized after the last compilation date. Only the first installation file uses the "factory" file in its name. Most wireless routers allow the first installation with OpenWrt / LEDE / SuperWRT from their original web interface, but there are also some that support firmware changes only via tftp.
After the first startup with SuperWRT, IP via dhcp will be provided in the 192.168.1.0/24 subnet, the wireless password is even "superwrt" and the first connection via ssh (or web on https://192.168.1.1) on the "root" is done without password (set it up urgently then!).
Updating OpenWrt / LEDE / SuperWRT is done using the "sysupgrade" file in the name itself from the LuCI web interface. You can choose to keep (most) the settings or rewrite with the default default options. Advanced users can update and ssh via the router using the sysupgrade utility (possibly with the -n option not to keep the settings).
It can be updated with SuperWRT firmware and directly from recent versions of OpenWrt or LEDE.
For details on returning to the official software, the best instructions are right on the OpenWrt and LEDE websites, but for most routers we have prepared special files that are written from the LuCI interface and return the original firmware.
If anything goes wrong, however, do not forget that most tp-link equipment can download a firmware file with a specific via tftp name and rewrite it without problems.
There are four differences between OpenWrt and SuperWRT : kernel without module unload support and no multicasting, a set of configurations and optimizations that apply automatically after the initial installation, a few pre-edited configuration files, and a multitude of additional packages installed/integrated in the firmware compiled by me.
All the information you find on the SuperWRT page, you have the page button in English, more information can give you Daniel Petre...
With my apologies, I just found
that appears to at least have the
.config files and the other files as well.
If you have any questions, leave a chat message for him https://superwrt.download/chat/!