If you don’t use ROS, then you don’t file a form 11 tax return, which is what the OP’s question relates to.
The OP filed a self assessment return via ROS, and if he wishes to amend it he can only do it via ROS (or by contacting Revenue, but they may (and should) simply direct him to the appropriate area in ROS).
I’m on my phone at the moment but will try to reply with a meaningful answer to the OP later, if nobody else does in the meantime.
Having submitted my 2017 Form 11 on Ros, I go to the Services/File a Return/ Form11/and choose the period 2017. I get a message that the file is being opened in Amend Mode.
3CC you should be able to find the same?