It sounds like your TCL TV is having trouble connecting to your Arris G18 router, but it works fine with a mobile hotspot. I would suggest to power cycle the tv, Unplug it for 5 minutes and plug it back in.

Restart your router G18 : Unplug the G18 for 30 seconds and plug it back in.

Try forgetting the network on TV: On your TCL TV, go to settings > Networks > WiFi. Find your Wi-Fi network, select it, and choose Forget Network. Reconnect by entering your Wi-Fi password manually.

If G18 has separate 2.4GHz and 5GHz networks, try connecting the TV to 2.4GHz. Also, change the security type to WPA2-PSK (AES) (some TCL TVs have issues with WPA3).

What tv model is this (not brand)?