Hi Leon,
Dodo are pre-paid month-by-month plans provider, and you need to activate the new SIM first in order to use it. If you haven't activated it yet, and don't want to use it, then you might be OK to just ignore it. Otherwise you need to login to their portal and do the necessary changes as per your decision e.g., cancel the plan, close account etc. At least, make sure Autorenewal is turned off, so you are not charged. Generally, if a SIM is unused/not charged for sometime, it will be automatically deleted by the provider meaning you loose the service and number.
If unsure, best to speak to their support:
https://support.dodo.com/