Good to know that TalkTalk Mail webmail works and the iPad is also receiving mail messages. Does the iPad connect to the mailbox via an Internet browser (webmail) or via an email software app (email client)? If via an email software app then that proves that the mailbox has not been limited to webmail only access and in turn that means the issue is with the iPhone.
What model iPhone is it? The required minimum iOS version is iOS 15. Have you done the usual power off and power on routine to clear any glitches from the email software?
Take screenshots of the incoming and outgoing email settings, image edit to obscure the first part of your email address and upload here using the picture icon above this Reply area. I'll take a look to see if there's a reason in the setup.
