DIAGNOSIS: A solid red light on the Internet globe icon means the router is powered on but cannot detect an internet signal from your modem. It's telling you the problem is upstream.
FIX: Don't panic—this is a cable or modem issue. Follow this sequence. 1) Check the physical cable connecting the modem to the router's blue WAN port. Make sure it's snug at both ends. 2) Look at your modem's lights. If its 'online' or 'signal' light is also off or flashing, the problem is with your modem or your ISP's service. 3) Power cycle both devices. Unplug both, wait 30 seconds, plug the modem in alone and wait for all its lights to be solid. Then plug the router back in. Wait for it to boot; the red light should turn white if the connection is restored.
IF THAT DIDN'T WORK: Bypass the router to test. Unplug the Ethernet cable from the router's blue WAN port and plug it directly into a computer's Ethernet port. If you get internet, the issue is with the router's configuration. If you still get no internet, the issue is definitely with the modem or your ISP—time to call them.
HEADS UP: If you recently changed internet providers or got a new modem, the router might be trying to use the wrong type of internet connection. You'll need to log into the router (tplinkwifi.net) and go to Advanced > Network > Internet. The 'Connection Type' should almost always be 'Dynamic IP'. If it's set to something else like PPPoE, that's likely the culprit.