First some basics…
1) Each user should always be using their own unique AppleID with their own iPad and iPhone. iOS/iPadOS devices are principally single-user devices that are not intended to be shared.
2) For an iPad to send and receive SMS/MMS messages or make cellular phone calls via an associated iPhone, both iPad and iPhone must be using the same AppleID - but this will only work if (1) is true.
Now, in more detail - covering a number of aspects, some of which may not apply to your particular issue…
The majority of Phone SIMs are not correctly provisioned for direct use with most tablet computers - and most will offer no service at all if installed in an iPad. If you have instead 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.
An iPad, whether a Cellular model or not, is not capable of directly accessing Cellular Voice or SMS/MMS Messaging features. An iPad with Cellular connectivity alone can only access IP-data services. An iPad is not, and can never be, a standalone substitute for an iPhone.
However, If paired with an iPhone, the capabilities of iPad significantly broaden to include access to Cellular Voice and Messaging services of the associated iPhone - relayed to the iPad over WiFi using Apple’s continuity features. For the most part, an iPad using Continuity is able to access Cellular calling features, of the iPhone, when both the iPad an iPhone are in close physical proximity and are connected to the same WiFi network - but this is not the end of the story.
Now, if your both your Cellular Carrier and your calling plan support WiFi Calling, your iPad can make and receive calls independently of the iPhone - even when the iPhone is switched off, elsewhere, or using a different WiFi network. Not all carriers support full WiFi calling.
More information about Continuity and the relevant Cellular services can be found here:
Continuity
Use Continuity to connect your Mac, iPhone, iPad, iPod touch and Apple Watch – Apple Support
SMS/MMS Messaging
How to forward SMS/MMS text messages from your iPhone to your iPad, iPod touch or Mac - Apple Support
WiFi Calling
Make a call with Wi-Fi Calling – Apple Support
When correctly configured, the iPad FaceTime App provides access to Cellular Voice Calling features - whilst Messages supports SMS/MMS messaging services.
Messaging:
Assuming that both iPad and iPhone are signed-in to iCloud with the same AppleID - to send SMS/MMS messages (green bubbles) from your iPad you must explicitly enable Text Message Forwarding for the iPad on the iPhone...
On your iPhone:
Settings > Messages >
- Text Message Forwarding - set to ON - and explicitly enable your iPad from the list of devices.
- MMS Messaging - set to ON (if MMS services are required)
Also ensure that Messages in iCloud is enabled on both your iPhone and iPad:
Settings > [Your Name / AppleID] > Messages - set to ON
WiFi Calling:
If your own Cellular Carrier and tariff support WiFi Calling, it is theoretically possible to configure WiFi Calling service for your iPad. After WiFi Calling is enabled and configured, your iPad can continue to make and receive cellular calls independently of your iPhone whilst connected to a WiFi network.
On the iPhone:
Settings > Mobile Data >
- WiFi Calling >
- WiFi Calling on This iPhone - set to ON
- Add WiFi Calling For Other Devices - set to ON
- Calls on Other Devices
- Allow Calls on Other Devices - set to ON
- [Allow Calls On] - explicitly enable your iPad from the list of devices
On the iPad:
Settings > FaceTime > Calls from iPhone > Calls from iPhone - set to ON
I hope this information and clarification is helpful in resolving your difficulties.