Given the following:
- iPad air with cellular modem (issued by T-mobile)
- T-mobile sim card from working telephone: re-allocated to iPad
What are the steps \ procedure to configure iPad's iMessage to receive text messages from SIM phone number?
You can't use SMS messaging on an iOS device (or any device for that matter) that doesn't connect to the GSM network.
SMS is part of the GSM services network; the "telephone" functionality of your device. It's the network that allows you to place a call to all phones including land lines. The iPad doesn't connect to the GSM network.
iMessage requires a data connection (WiFi or cellular data like LTE) because they are sent and received through a server over the TCP/IP protocol.