Verify that both the iPhone and the Apple TV box are connected to the same home network. If your iPhone is only connected to cellular, then it won’t be able to connect to the Apple TV.
For Wi-Fi, verify that all your devices use the same SSID network name. Creating separate network names for the 2.4 GHz and 5 GHz band will result in separate networks that cannot ‘talk’ to each other.
Verify that bluetooth is on for the iPhone during discovery.
Get help with Wi-Fi networks on your Apple TV - Apple Support
Verify that your neighbors aren’t interfering with their Wi-Fi in the same or overlapping Wi-Fi channel.
Verify that wiring or metal objects or other electrical devices aren’t causing interference.
Resolve Wi-Fi and Bluetooth issues caused by wireless interference - Apple Support
To pair your original remote again, try the pairing procedure again. If your Apple TV remote isn’t working - Apple Support
If your original remote is out of battery charge, then charge it again. Charge your Siri Remote or Apple TV Remote - Apple Support
If your original remote is broken, consider buying a replacement remote. Siri Remote - Apple TV accessories - Apple Store