I have moved you onto your own thread rather than you jumping on some other person's thread which only causes confusion as to who the answers/replies are for.
Because you were not a BT Broadband customer or paying for your email account through a BT Premium email account the information you were given was correct about your account being down graded to "Basic" email account.
See link
https://www.bt.com/help/email/bt-email-products
Now that you are paying for a BT Premium email account you should be able to use an email client/app.
It may be the case that to "kick start" your synchronisation between webmail and the email client/app on your device you will need to carry out a password change.
To help this work you should stop your devices from checking for email. The easiest way of doing that is to turn them off if you still have the account on your device or add a couple of characters to the email address so that it is not checking your account. If you have deleted the account and it is no longer on your device do not try and add it at this time.
Once you have done that, change your email password and check that you can access your email via webmail (on a browser).
If that works you should wait at least an hour and preferably longer to allow the servers to sync and then add the new password to your device, remembering to remove any characters you may have added and check it works. If it does then move onto the next device if you have one.
When entering the settings, make sure that in the Outgoing server you do not use the "use same as incoming" for the email address and password. Enter them manually.
If the above does not resolve the problem, when you change your BT password on an iPhone/iPad/Apple device it may only change for INCOMING mail.
To change the outgoing mail password you have to go into Settings; then scroll down to Passwords & Accounts or on older devices Mail, Contacts, Calendars. From there select the BT email account and select the account where it is listed under IMAP.
You will find the details for the account and can change the password for the Incoming server there, the outgoing mail server only shows SMTP and mail.btinternet.com You have to tap on that Primary Server in order to find where the Outgoing password is hidden so you can update it.
When setting up the email account on your device you are best to set it up manually, using "other" and ensure that you are using the correct settings as per this link.
https://www.bt.com/help/email/manage-email-account/manual-settings/what-are-the-settings-for-outgoin...
