To fly IFR in any aircraft requires a pilot to hold an IR which is valid for that aircraft.
You state you have an IR specific to the ATR Type; this does not, therefore, allow to fly any other aircraft IFR that does not fall within the same Type Rating.
SEP (Land) is a Class Rating, you would therefore need an IR applicable to that Rating.
This would be annotated on your Licence as 'IR-SP-SE', where SP = Single Pilot and SE = Single Engine.
Note: Bose-X gives valid information for ease of maintaining an IR-SP-SE once you have one.