Entware-NG is a modern alternative to Optware. Originally designed for OpenWRT, it is also usable by other firmware platforms such as Asuswrt-Merlin based routers
Works only on usb drives formatted as EXT2, EXT3 or EXT4 for Arm devices
1 - Download RMerlin firmware from here, go to firmware upgrade page and upload normaly like a stock one
2 - Go to Advanced_System_Content, check "Enable JFFS custom scripts and configs" and "Enable SSH" (LAN-only is recommended), press Apply then Reboot
4 - After reboot, connect to router with putty terminal and type:
wget -c -O entware-ngu-setup.sh https://bit.ly/2xZTaVp
chmod +x ./entware-ngu-setup.sh
Info: This script was created by ryzhov_al and modified by TeHashX.
Info: Thanks @zyxmon & @ryzhov_al for New Generation Entware
Info: and @Rmerlin for his awesome firmwares
Info: This script will guide you through the Entware-NG installation.
Info: Script modifies only "entware" folder on the chosen drive,
Info: no other data will be touched. Existing installation will be
Info: replaced with this one. Also some start scripts will be installed,
Info: the old ones will be saved on partition where Entware-NG is installed
Info: like /tmp/mnt/sda1/jffs_scripts_backup.tgz Info: Looking for available partitions...
 --> /tmp/mnt/sda1
=> Please enter partition number or 0 to exit
5 - Choose a partition where Entware-NG should be installed, in this case is only  --> /tmp/mnt/sda1
Info: /tmp/mnt/sda1 selected.
* Warning: Found chrooted-debian installation, stopping...
* Warning: stopping running services...
* Warning: Found Entware installation, saving...
* Warning: Found Old Optware installation, saving...
* Warning: Found Old Optware-NG installation, saving...
* Warning: Found previous installation, saving...
Info: Creating /tmp/mnt/sda1/entware folder...
* Warning: Deleting old /tmp/opt symlink...
Info: Creating /tmp/opt symlink...
Info: Creating /jffs scripts backup...
tar: removing leading '/' from member names
Info: Modifying start scripts...
Info: Starting Entware-NG deployment....
Info: Creating folders...
Info: Deploying opkg package manager...
Info: Basic packages installation...
Updated list of available packages in /opt/var/opkg-lists/entware.
Installing ldconfig (1.0.8-1) to root...
Installing findutils (4.5.14-1) to root...
Installing libc (1.0.8-1) to root...
Installing libgcc (4.8.5-1) to root...
Installing libssp (4.8.5-1) to root...
Choose swap file size (Highly Recommended)
3. 2048MB (recommended)
4. Skip this step, I already have a swap file / partition
or I don't want to create one right now
Enter your choice [ 1 - 4 ]
6 - Choose desired swap size (2048MB is recommended)
Info: Creating a 2048MB swap file...
Info: This could take a while, be patient...
2097152+0 records in
2097152+0 records out
Setting up swapspace version 1, size = 2147479552 bytes
Press [Enter] key to continue...
Congratulations! If there are no errors above then Entware-NG is successfully initialized.
Found a Bug? Please report at https://github.com/Entware-ng/Entware-ng/issues
Type 'opkg install <pkg_name>' to install necessary package.
7 - The script created a directory named "entware" in the selected partition, type cd /opt
admin@RT-AC66U:/tmp/mnt/sda1# cd /opt
8 - Now Entware New Generation is installed on your asuswrt router, it's time to install some packages, ex. nano text editor:
opkg install nano
admin@RT-AC66U:/tmp# opkg install nano
Installing nano (2.4.2-1) to root...
Installing libncursesw (6.0-1) to root...
9 - If you want to start / stop a single installed package, type "/opt/etc/init.d/SXXpackagename start / stop"
9b - To start / stop all installed packages, type only "services start / stop"
You can install packages from here: ARM 32bit, ARM 64bit, Mipsel
Thanks @zyxmon and @ryzhov_al
REVIEW (CLICK ON THE STARS TO RATE!)