my tp-link AC1200 shows blue lights for the 5G but not for the 2.4 G

To connect  tp link router  to the network turn on your  tp link and press reset key for 30 seconds. Go to your computer or phone and open browser and type 192.168.1.1 in the top url bar. It will ask for your login information (default username and password is admin for both). Then it will open setup page. Just follow the instructions and your setup. Once the setup is done then you can easily connect your router with the both 2.4 G and 5 Ghz  network.

If the TP-link router is showing you a blue light on the 5Ghz but not on the 2.4Ghz the possibility is that the setup was not successful follow the steps below:

• Make sure the router is On, Press and hold the reset button down for 30 seconds
• Let the router reboot
• Connect your computer to the Tplink Router wirelessly or via Ethernet Cable
• Pull up a browser and the top on the address bar type in 192.168.0.1
• You will get to the Tp-Link Router Setup screen follow the onscreen instructions to complete the setup
• once the setup is complete connect your devices to both 2.4 and 5Ghz network.

If still, you have the same issue. reset the router again

This is only because  at that time your devices are connected with only 5 G band . If you connect your device with 2g band that you see 2g light will lit .