For future information : if you are travelling to Dominica, the ATM at Republic Bank
in Roseau provides fee free withdrawal of Eastern Caribbean Dollars. More remote villages won’t have ATMs or card payments and you will need cash to get by …
Edit…
National bank of Dominica ATMs do not work (not just with monzo)… The machine will say it’s not possible to complete the transaction… and monzo will show the withdrawal, followed by an immediate refund. This eats into your daily ATM allowance and can incur Monzo ATM fees even though the machine does not dispense… 
ATM hell! But fortunately monzo refunds the fees!!
You should be aware that Dominica is mainly a cash economy, but most large shops in large towns will accept cards. There seemed to be a number of different methods for doing this. Most shops will ask for ID (driving licence will do). In addition some shops swiped the card and required a signature, others took the card and used it contactlessly, and some used chip and pin…