Hey JRS24 - If you turn on MTD for that client in the Advanced Settings for the client - Enter all the neccesary information and it should still present the return dates that HMRC are expecting, which would be the already "Filed" Return.
If it doesn't, then we'll need to unfile the return for the client, which you'll need to call us for us to arrange which you can do through your pro-advisor contact as normal.