Your perception of the function, significance/importance of a “registered number” upon your WiFi+Cellular iPad is unclear…
The majority of mobile phone SIMs are not correctly provisioned for use with tablet computers - such as iPad. If a mobile phone SIM works at all, it may not provide access to the full range of cellular services. This is not a limitation of the iPad, but of the Cellular network provider provisioning of the SIM.
If using a physical SIM with iPad, you will require a DataSIM explicitly provisioned for iPad or mobile broadband (a.k.a. MiFi).
Be aware that an iPad, whether a Cellular model or not, is not capable of directly accessing Cellular Voice or SMS/MMS Messaging features. An iPad with either WiFi or Cellular connectivity alone can only access IP-data services. If you have added a DataSIM and Cellular plan to a WiFi+Cellular model of iPad, the number associated with the SIM card is only used to identify your account for billing purposes. This number is not used for making calls or sending messages.
With benefit of the above information, perhaps you can now provide more information about precisely what you are attempting to achieve.