I haven’t had a chance to try it on mine but, if you’re going to modulate Osc1 level, you’ll need to turn down Osc1 level in the program menu (or by holding down Osc 1 and turning the Value encoder) so it’s not maxed out already, or else any positive mod amount won’t do anything.
Alternatively, use a negative modulation amount in the matrix.
Next, true ring modulation is through zero, so using the matrix in this way won’t give you true ring modulation, but just amplitude modulation.
Lastly, there is a Ring Modulation effect in the effects section.