Contacts

I have a Windows 10 laptop, over 10 years old, on which I have set up my myname@btinternet email and have about 600 contacts. My wife had an oldish ipad and we both have iphones. We had set up a separate email account (a sub account on mine) for her, hername@btinternet and this was used to register her iphone. We could send & receive emails on all 4 devices to all 600 contacts.

My wife got a new ipad for Christmas and this only gives a very reduced contact list, being those contacts she has set up under her name. We have been to the apple shop and they could not help.

How do we make the full contact list available on all 5 devices?

Thanks for any help you can give.