BIG WARNING: Increasing your router to it’s maximum power can reduce it’s life expectancy, cause problems for your neighbours, can be illegal depending on your location and in some cases even has the potential to cause actual dangerous situations since RADAR can share this frequency!

Taking the above warning into consideration, the allowed transmit power depends on your location and any transmit licenses you might have. With many people living in modern houses which are essentially faraday cages not a lot of harm can be done by increasing the power a bit. But due note that this might not be legal in your country so please check how much you are allowed to transmit using this table.

ASUS RT-AC68U

The first thing you need to do is install the Merlin firmware version 374.43_2, newer ones won’t work! You can download the firmware from Merlin’s site: http://asuswrt.lostrealm.ca/download

Secondly, after installing this you will need to connect either through telnet or through ssh. Here you can enter the following commands but don’t blindly enter them! The last few tell you the transmission power and you probably don’t want it to be 500mW, but these are (as far as I know) the maximum supported:

[bash]
nvram set 0:regrev=0
nvram set 1:regrev=0
nvram set 0:ccode=#a
nvram set 1:ccode=#a
nvram set pci/1/1/ccode=#a
nvram set pci/1/1/regrev=0
nvram set pci/2/1/ccode=#a
nvram set pci/2/1/regrev=0
nvram set regulation_domain=#a
nvram set regulation_domain_5G=#a
nvram set wl_country_code=#a
nvram set wl_country_rev=0
nvram set wl0_country=#a
nvram set wl0_country_code=#a
nvram set wl0_country_rev=0
nvram set wl0_reg_mode=off
nvram set wl1_country=#a
nvram set wl1_country_code=#a
nvram set wl1_country_rev=0
nvram set wl1_reg_mode=off
nvram set 0:maxp2ga0=114
nvram set 0:maxp2ga1=114
nvram set 0:maxp2ga2=114
nvram set 1:maxp5ga0=114,114,114,114
nvram set 1:maxp5ga1=114,114,114,114
nvram set 1:maxp5ga2=114,114,114,114
nvram set wl_TxPower=500
nvram set wl0_TxPower=500
nvram set wl1_TxPower=500
nvram commit
reboot
[/bash]

Note that the #a here is the country, #a is the “wildcard” country which enabled all channels supported by the hardware.

The TxPower are the ones you need to change to set the transmit power and the nvram commit permanently saves these settings to the router. After that the reboot command will apply everything for you.

I should note that this firmware has some issues with a crashing web interface, luckily that’s easily recoverable through ssh. Just enter this command through ssh and it will be up and running again:
[bash]
service start_httpd
[/bash]