How do I set up linksys E900 wireless-N300 router without the instruction

Linksys router is the best network service provider between another router service provider, there are features provide high-speed internet, multiple connectivities, easy for installation and etc.
To troubleshoot Internet connectivity issues:
First of turn off your router and connect modem directly with your computer.
Login your router by following login manual steps.
Click the Smart Wizard Test button to access the NETGEAR online test page.
Under Router Status, and check your router has a valid IP address. For more information, see your router user manual setting.
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The password is wrong, or the phone does not support the protocol on the wireless router.

It's a wireless router, so you may not need to use a port at all.

You can connect to any of the ports numbered 1 through 4. The yellow Internet port should connect to your outside network. (Cable modem or router or Ethernet port from your Internet service provider.)

Reset the factory to "factory" settings.
This reverts to the original ID/password.
Then, you can "setup" the router (ID/password/SSID/security).

You could try to login to the router and check the WAN connection, ensure that the connection time to idle is set at '0', this will make sure that it is up all the time.

Are you using wifi ?

Turn the router on.
Connect an Ethernet cable from your computer to one of the 4 'LAN' ports.
On your computer, hold down the "Windows" key, and press/release the 'R' key.
In the mostly-black window that appears, type 'cmd' and press ENTER.
Type 'ipconfig'.
Read the output, looking for the IP-address of the "gateway".
Start "Internet Explorer".
Hold down the CTRL key, and press/release the 'o' ("oh") key.
In the window, type http://192.168.aaa.bbb
where 'aaa.bbb' is replaced by the IP-address that was listed for the "gateway".
Login to router.
Click the "Setup" button, and follow the instructions.

Google-search "reset Linksys e900" to restore it to "factory" settings, including the original ID/password. Then, start over to setup/configure your router, including setting a new password.

As per its user manual (p.25), admin for both username and password

Good luck

If you are not able to use wirless signal of your WI-FI router you must add another E900 AP and use same SSID with Different Channel Configuration.