If you mean that incoming messages arrive in the Inbox and you see them but then those same messages disappear then that suggests that mail is being downloaded and deleted by a mail collection service or other device connected to your mailbox as a default POP3 email account.
To lock out any unauthorised devices, upgrade the password for the mailbox and initially test only with TalkTalk Mail webmail to make sure the mailbox is working as expected.
The Reset password now button in the help article Changing your email password will take you to password reset and recovery.
If you have set up password recovery options, in advance, that you recognise from the *****hints, then you can send your alternate email address a link or your mobile a 6-digit code to authorise a password reset.
Select here: Sign in to TalkTalk Mail
Enter your full TalkTalk Mail email address, select Continue and enter the password, select Sign in.
Check there are no Rules operating on the Inbox messages.
Once you know mail is being received normally and not diverted from the Inbox you can resume using the mobile email app as an IMAP email account.
If you need guidance on any of the above then just say.
