In fact, Otherworld is not part of any city, so it is an area that is not affected by any city's increased respawn rate.
To find out if an area can be affected by the increased respawn rate, simply go to the area and open the map in Cyclopedia. In Navigation you can check which city that area belongs to. If no city name appears, then unfortunately the area cannot benefit from the increased respawn rate.
The image below was taken while I was in Otherworld and shows that it is not part of any city:
It is worth noting that the rapid respawn event affects all areas, including those that are not part of any specific city.