After installing OpenWrt on your router, you will be able to access it via SSH without having to configure anything else because it is enabled by default on port 22 for the root user + password.
If you need to make changes the SSH access configuration, which is handled by the dropbear subsystem, go to ‘System > Administration‘.
So, just use your favorite SSH client or type ssh root@192.168.1.1 in a Terminal window if you are on Mac or Linux (for a router whose address is 192.168.1.1).
The first time you ssh into your router, you will probably see a warning about the RSA key fingerprint. If you are certain this is the address of your OpenWrt device, simply type yes and press Return. Then enter the password you have given to your router, or press Return if this is the initial setup.
ssh root@OpenWrt
The authenticity of host 'openwrt (192.168.1.1)' can't be established.
RSA key fingerprint is SHA256:######################################.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added 'openwrt,192.168.1.1' (RSA) to the list of known hosts.
root@openwrt's password: *******
BusyBox v1.28.4 () built-in shell (ash)
_______ ________ __
| |.-----.-----.-----.| | | |.----.| |_
| - || _ | -__| || | | || _|| _|
|_______|| __|_____|__|__||________||__| |____|
|__| W I R E L E S S F R E E D O M
-----------------------------------------------------
OpenWrt 18.06.4, r7808-ef686b7292
-----------------------------------------------------
SSH clients
If you use a Mac or Linux you will not require any special software to access your router via SSH. Otherwise, if you use Windows you can use different terminal emulators such as cmder, Putty or WinSCP that allow you to SSH into your router.