There is no sms/mms in my ipad settings to activate.....can only imessage...help!

I did read the article. I called my carrier and Apple before getting the ipad to be sure it was doable. They both said yes. I have had an ipad on my account before and I have had one without. This is for my mother(think of it as for your kid). I can use an ipad over wifi and text android or Mac(imessage for a Mac). I have a sim card but either the carrier or Apple should have the sms/mms in the settings under messages. It's not there! it's on all other Mac devices but not the one I need it for. With Apple and Carrier completely stumped for 3 days I need to who enables the sms/mms? I got locked out of my account with ATT and they couldn't send the verification to my phone! After 3 techs it was fixed and then my account page was really different. After more calls to both Apple and ATT 'my' ipad that was working fine yesterday said it had no cellular...ugh. Also never showed on the account till I dug deeper. Now from my own account I activated it. I know I will have to start the calls again but mom's ipad still shows but no sms/mms. I only got her the cellular to be able to find her because she lives alone. It may be easier to get her wifi instead but the messages settings should show - who is responsible or is the ipad defective?

Apple had me set up a separate Apple ID for mom's ipad. it does show. up on my account and I changed the name of it. The Techs at ATT could see both ipads - I could only see my mothers. My ipad air was working fine. Until yesterday - the cellular just disappeared. I couldn't spend hours on the phone so I added it on myself manually to account. Now I have to call them but still not understanding why there is no sms/mms anywhere on her device. Who makes that happen or is device defective? Ready to get rid of both of them and go back to android....ugh. BTW I am on an Island and we have no ATT here. I had to go to the mainland to get the sim cards put in and activated.

messages is no where to be found in her icloud account and in settings there is no sms/mms under messages or anywhere! I have had 2 Apple techs sign in and say it should be there and ATT should do it. Even when I call ATT with Apple we get disconnected. I just want my mother (who is 80, now lives alone and can't figure out a cell) to have something bigger , that she can use to message or eventually facetime with other people. See the outside world. My friends had them with there kids. I was told this could be done but do I have a bad ipad , the system changed, Apple needs to fix or ATT can add the sms/mms??? Who controls this? I get a different answer from Apple and ATT.