That's a very small tank if it's 35 litres. Are you sure it's litres? You don't need a distinct UV source for frogs.

Regarding the heat mat, in my experience they are not a good idea for heating frogs, however if they are used on the side of the tank, rather than the bottom, then they are good for supplementary heating (such as at night in winter). For day time heating I would use a normal day light bulb in a light fixture on top of the terrarium.