I'm not very conversant with celsius temps, but when I have a hard time getting my incubator up to temp, I snug it in with towels--just avoid covering the vent(s). You may also consider turning up the thermostat in the room. My incubator needs a 65-70° Fahrenheit room, but prefers 70-75°. Holderread's hatching instructions indicate that duck eggs can be expected to hatch (approximately) on days 26-29, depending on the breed. Lower temps during incubation will mean a later hatch and possibly other problems, but I don't know much about that. Mostly people don't seem to have noticeable negative effects other than a late hatch.
I would not worry. Ducks take their own sweet time and lots of it.
@FortCluck has a May Hatch-a-Long going right now. You ought to join in. She's got quite a lot of experience with ducks and can help you more than I. Here's a link:
https://www.backyardchickens.com/threads/may-2020-hatch-a-long.1359364/