If you meet all the requirements, login to router with putty terminal and enter:
Choose a username and a password for your database and replace those in red color
Exit with Ctrl-C
Go to 192.168.1.1:81/wordpress/wp-admin/install.php
Choose a language
Click on Create a Configuration File
On the next page click on "Let's go!"
Now fill in with user_name and db_password created earlier with mysql commands.
Change Database Host from localhost to 127.0.0.1
On the next page, click on "Run the install"
Choose a Site Title, username, password (don't have to be the same as for mysql)
Enter your email adress
Press "Install WordPress"
Customize appearance, add widgets, menus, plugins...
Now access your blog on 192.168.1.1:81/wordpress to see how it looks.
Create an asus, no-ip or dyndns dynamic dns account and the blog can be accessed from your_dns_account/wordpress or your_wan_ip_adress/wordpress
Create a backup of your blog and mysql database directly from the router, (if your usb disk has a label, change sda1 with usb label)
If something goes wrong, just restore with:
I hope everything goes smoothly for you, if not, don't hesitate to ask here.
See HERE a terminal recording
Youtube video HERE
Tested at 14/03/2016 on RT-AC68U, wordpress v4.4.2