When iPhone has data and iMessages turned off, can’t receive group texts from other iPhones. Can send group texts, can send/receive individual texts. But group texts go as iMessage and never arrive as sms.

If you have ANY Apple devices with iMessage turned on, and someone sends you an iMessage, it will be delivered to the device(s) with it enabled, but not to your phone under the circumstances you describe.

Group messages that include recipients who do not have iMessage enabled are sent as MMS messages, NOT SMS text messages. MMS requires cellular data to be active, so if that's how they are being sent, you will not receive them if you have data turned off.

SMS messages use the voice connection, not the data connection, so you will be able to send and receive SMS messages.