Hi!
There are several things you might try to solve the SIM card problem on iPhone 6. Some common solutions include restarting your iPhone or checking if there are any manager settings updates.
To restart your iPhone:
Long press the Side (power) button and the Home button simultaneously until the Apple logo appears
To check for manager settings updates on your iPhone:
Settings > General > Info. If an update is available, you will see a prompt to select OK or Update.
Another thing you can do is put another SIM in the iPhone 6 and find out if the problem is with the phone or SIM.
If the problem persists, contact your carrier and ask for information, including compatibility of your new SIM with iPhone 6. Alternatively, contact Apple Support.
I hope I helped you!