Where are those messages?

If they are on the server it is very simple, just set up the account and they will be downloaded no matter how old they are.

If they are no longer on the server and are only on that iPhone, or should be, unfortunately I am afraid that if they do not appear in the list of messages there is no way to recover them, at least in a normal way.
Maybe by jailbreaking and with some special application you can access that data, if they are still on the phone.

If they are still on the phone.