If your mobile SIM is already activated and the mobile data is ON, you should not have a problem accessing Facetime. Kindly ensure that your mobile data is enabled if activating thru our network. Have you tried to your Facetime using a WiFi connection?
It seems that this is not an issue related with your mobile data nor Wi-Fi. I recommend that you contact Apple Support regarding this error. You can also check https://support.apple.com/en-ph/HT201422.