Do you really need to make bank transactions? I don't know that I'd trust the wifi on the ship to be secure enough for that?
Call AT&T and ask them about your options for the ports that you are going to. ATT using GSM network technology which is much more compatible with the rest of the world. You likely will be able to use your cell in ports. I just contacted Sprint about this and I can use it in all of my ports next week. You just need to find out if there is cost and get your phone set up so that it will roam internationally. They will help you with that.
On ship though, you won't have ATT cell service. Some ships have cell service. It's the Maritime service. https://www.wmsatsea.com/faqs.html But it's stupid expensive. You probably do not want to connect to this. You will want to put your phone in airplane mode when the ship leaves ports.
So when you are at sea, your best option would be the VOOM wifi service. Then your phone can get on the internet. If your phone is capable and ATT allow, you can set up wifi calling. Then you can make and receive calls over wifi and it acts just like normal calls. Texts however are not sent via wifi I don't believe. So you may want to consider a different messaging app and tell family to use that. Mine will be contacting me via FB messenger.
I can't advise on an iPad. I have one, never use it and won't be bringing it on the cruise. My phone is sufficient for what I will be doing. I hope that helps and if I gave any misinformation I hope someone corrects me. :)