As understood, for your iPad, you objective is to disable iMessage (blue bubbles) - but still wish to be able to send and receive SMS text (green bubbles).
First, for the benefit of clarity, it may be helpful to outline some basics. You may already understand this, but for the benefit of other readers, it may be useful background…
Messages App
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.
However, when used along with an iPhone, the capabilities of iPad broaden to include access to the Cellular Voice and Messaging services of the associated iPhone - relayed to the iPad over WiFi using Apple’s continuity features.
As such, without an iPhone, the iPad’s Messages App can only access iMessage (blue bubbles). An iPhone without a Cellular plan is similarly constrained - but with a Cellular plan (as for an iPhone is likely), you have access to SMS/MMS text messaging (green bubbles) for both iPhone and your iPad.
The Messages App will try to use (i.e., prioritise) iMessage (blue bubbles) between Apple device users whenever possible - only using SMS (green) when not connected to a WiFi network that has internet access; SMS is always used when exchanging messages with users of Android or non-Apple devices - or where a WiFi network with an internet connection is not available.
Messages App setup
Set up Messages on iPad - Apple Support
Set up Messages on iPhone - Apple Support
For the iPad to access SMS/MMS (green bubble) messages via the iPhone, both iPad and iPhone are signed-in to iCloud with the same AppleID - and you must explicitly enable Text Message Forwarding for your iPad on your iPhone...
On your iPhone:
Settings > Messages > Text Message Forwarding - set to ON - and explicitly enable your iPad from the list of devices.
To optionally use MMS, this must also be enabled on the iPhone:
Settings > Messages > MMS Messages - set to ON
Also ensure that Messages in iCloud is enabled on both your iPhone and iPad:
Settings > [Your Name / AppleID] > Messages - set to ON
When setup correctly, both iPad and iPhone can independently use iMessage - and both can use SMS/MMS via the iPhone. For your iPad to access SMS/MMS, this will always be contingent upon routing these messages via your iPhone. To reiterate an earlier point - an iPad, whether a Cellular model or not, is not capable of directly accessing Cellular Voice or SMS/MMS Messaging features without the iPhone.
Returning to your objective - namely to use just SMS/MMS from the iPad - this might be possible by signing out from the iMessage service, but not iCloud, on your iPad. To sign-out from iMessage on your iPad:
Settings > Messages > Send & Receive > AppleID > Sign Out
NB: The AppleID field is not immediately obvious. It is found, in very small text that is easy to miss, immediately below the settings entitled Start New Conversations From.