Please try the following:
Connect to the same network:
Your phone or tablet should be connected to the same network as your smart TV. Using two fingers, swipe down from the top of your phone's or tablet's screen to open the Quick settings panel, and then touch and hold the Wi-Fi icon to open your device's Wi-Fi settings. Here you can check if it's connected to the right network and can change it if needed.
Allow permissions on your TV.
Using two fingers, swipe down from the top of your phone or tablet’s screen to open the Quick settings panel. Tap the Smart View icon, and then select your TV. If your TV requires permission, you will need to select ALLOW on your TV.
Check your TV’s software:
Make sure your TV’s software is up-to-date. If you have a Samsung TVcheck the available software updates from the Settings menu. Consult your manufacturer if you have another brand TV.
Adjust the aspect ratio on your phone or tablet.
If your phone or tablet’s image looks too small on the TV’s screen, you can try adjusting the aspect ratio in Smart View. On the phone or tablet, tap the blue Smart View icon, and then tap Phone or tablet aspect ratio. From here, you can choose your desired settings.
Restart your TV:
Restart your TV by pressing and holding the TV's Power button. Then, turn the TV back on again.
You can also unplug the AC power cord, and then plug it back in.