Welcome to the community!
If you have a post-paid plan, Fido Roam is likely already enabled. Any usage (make/receive calls, send text messages, and any data usage) will trigger the daily fee which will allow you to use the services for 24 hours. Note that incoming SMS are not charged; however, the data within a MMS would be considered roaming data. The rate for the Dominican Republic is shown below:
~taken from here.
If you wish to travel using the pay-per-use rates, you will need to contact customer service to opt-out of Fido Roam. Alternatively, you might consider sending a PM to @FidoSolutions. Once they verify some information, they will also be able to access your account.
If you do not intend on using any data, you might consider disabling your phone's data and roaming data. Note that disabling roaming data does not prevent roaming for voice and messaging.
If you do not intend on using any of your services, you might consider enabling Airplane or Flight mode for the duration of your trip. That setting should prevent the phone from connecting to the networks. You could then manually enable Wifi when required. Note that if your phone is capable of Wifi-calling, certain usage will also be considered roaming.
Hope this helps 😀