Relay Contact Rating (DC Power) at different DC voltages
You need to comply with all of :

From the datasheet:


Max. Switching Voltage:250VAC/30VDC
Max. Switching Current:10A
Max. Switching Power:300W 2500VA


When NOT switching (so steady state on or off) the ratings are:


Contact Rating(Res. Load):10A 250VAC/30VDC


And no, you cannot trade one for the other, so 10A max and 30V max independently.

Note also that the contact rating is for a resistive load, it will be much lower for switching a capacitive or inductive load.