Hi
I know it's been a month since you asked but I managed to fix my router yesterday. Based on the post below I tried and succeeded to increase the txpower up to the legal limit on my risk of bricking the device.
First of all I ssh root on my mi-4C and did an "cat /proc/mtd"
_______ ________ __
| |.-----.-----.-----.| | | |.----.| |_
| - || _ | -__| || | | || _|| _|
|_______|| __|_____|__|__||________||__| |____|
|__| W I R E L E S S F R E E D O M
-----------------------------------------------------
OpenWrt SNAPSHOT, r15421-498fb1b8aa
-----------------------------------------------------
root@OpenWrt:~# cat /proc/mtd
dev: size erasesize name
mtd0: 00020000 00010000 "bootloader"
mtd1: 00010000 00010000 "config"
mtd2: 00010000 00010000 "factory"
mtd3: 00010000 00010000 "crash"
mtd4: 00010000 00010000 "cfg_bak"
mtd5: 00100000 00010000 "overlay"
mtd6: 00ea0000 00010000 "firmware"
mtd7: 002052ec 00010000 "kernel"
mtd8: 00c9ad14 00010000 "rootfs"
mtd9: 00a10000 00010000 "rootfs_data"
root@OpenWrt:~#
So far so good. Then downloaded mtd2.bin(factory). I used Luci, System>Backup/Flash Firmware> Save mtdblock contents>factory
Opened file in Hex editor and changed 14 values to 0xFF as in screenshot.
Then save as mtd2.bin and upload to router /tmp using scp
cd path-to-mtd2.bin
scp mtd2.bin root@192.168.1.1:/tmp
Unfortunately trying to write /tmp/mtd2.bin to factory partition didn't work
BUT... risking again I used kmod-mtd-rw
opkg update
opkg install kmod-mtd-rw
...and then the magic words I found on the net:
insmod mtd-rw.ko i_want_a_brick=1
Then using mtd I wrote to flash:
mtd write /tmp/mtd2.bin factory
I had to do that a few times before success.
Did a reboot and now I am able to set txpower from 0 to 20dbi.
I did simple tests with wavemon and it works!
Sorry but I can only post one screenshot.
I didn't test if flash partitions remain unlocked for writing after reboot!
Also I have to do something so my command lines show properly 
I hope someone finds my post usefull.