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Unfortunately, I'm not sure anyone would be able to provide a satisfactory answer. Like all mobile providers outside of the Dominican Republic (DR), Fido would rely on the local provider infrastructure to provide access when in Country. In addition, there are many factors which can affect cellular reception (ie proximity to cellular towers, device used, possible sources of interference,.. etc).
When I was last in Punta Cana, I don't remember having any issues with cellular service. However, we only stayed at one location and did not travel outside of our resort. My experience with cell phone service might not be the same as someone else's at different locations within the DR (or yours at your intended location). The only way to determine how well your phone would work in the DR would be to test it where you intend on using it. I understand that answer might not be very helpful, however, cellular reception does depend on a particular device at a particular location.
That said, you might consider checking other sources regarding local cellular services at your intended destination(s). As mentioned previously, foreign providers would roam on local infrastructure. Information regarding the local cellular services at your destination(s) would be more useful than isolated experiences of vacationers visiting possibly different locations than yourself.
In addition, cellular coverage maps within the DR might also be helpful. There may be other coverage maps as well.
You might also verify your device has compatible bands/frequencies with the local networks. You can verify those bands/frequencies here and here. You might consider verifing those compatible bands/frequencies with other sources.
Alternatively, if your device is supported for Wifi-calling (and you readily have access to Wifi at your destination(s)), that may also be an option. You should note that some Wifi-calling usage would also be considered roaming and incur Fido Roam charges.
An additional consideration... What plan do you have? You should note that some Fido plans are not eligible for roaming. You would need to verify your plan is eligible for roaming. If you have a post-paid plan, you should note that Fido Roam is automatically enabled. If you wished to opt-out of that feature, you would need to contact customer service. In addition, they can also be contacted via Live Chat, Facebook, or Twitter. Those methods can be accessed via the contact page posted above. You should also note that if you opt-out of the feature, any usage would be on a pay-per-use basis. You can view the rates here.
Hope this helps 😀