Where did you see the error? Provide a screenshot pls. You may contact your ISP to check the username and password for internet connection, reset and reconfigure it...

No Wi-Fi Icon or you see the icon but cannot detect or connect to the Wi-Fi network? How often does it happen? Have you tried to reset it to facotry defaults and reconfigure it? Is it a brand new...

How did you connect the printer and router? By ethernet port, USB port or wirelessly? There is a Print Server Compatibility List. Have a check. http://www.tp-link.com/common/compatible/print-server/

Since you can connect to the interface of WR740N, check if there is WAN IP on Status page. Besides, go to System Log and check if there is any error there. We cannot confirm it is modem issue or...

When you cannot access internet wirelessly, can you log in web interface of WR740N? If not, how about status of Wi-Fi connection on wireless clients? How many wireless clients are connected in total?...

You tried to install the Windows 8 or Windows 8.1 driver manually?