Data not working in Saint Lucia

you need to enable data roaming in your phone and select the network operator in automatic

Try to manually select the other operator in St Lucia, Digicel, I believe. See if you get service there.

Check your Access Point (APN) settings on your phone. This may or may not be possible depending on what phone model you have.

The official Fizz FAQ is here: https://fizz.ca/en/faq/what-are-fizz-network-settings-apn

iPhone
Check if you have a menu entry
Settings→Cellular→SIMs(Select Fizz)→Cellular Data Network
Most likely, you won't have this entry as Fizz is telling your phone to hide it.
While connected to Wi-Fi and running iOS 18.3.1, you can also try to get a manual carrier settings update, by going to Settings→General→About and wait for a Carrier Settings Update prompt which may or may not appear. If none appears, check if under Carrier it says Fizz 62.0. I believe this to be the most recent profile.

Android
Log into your Fizz account on the web.
Go to My plans → Mobile (Your phone number) → Manage plan → Everything else → Advanced parameters → Fizz network parameters → Send to my phone.
If that doesn't work or it tells you to do the setup manually, you have to find the APN in your phone menu.
Where exactly to find this option depends on the phone vendor, Android version and cell network operator. Something like this:
Settings -> Connections -> Mobile networks -> Access Point Names
Settings -> Network & internet -> Mobile network -> Advanced -> Access Point Names
The behaviour of this menu is somewhat unpredictable because it again depends on phone vendor, Android version and cell network operator. You may see the profile in use, but can't edit it, or you may see some profiles that are not in use and the used profile is hidden. You may be able to add profiles or you may not, depending on the data you enter, which is again due to Fizz restricting what a user can do.
These are the settings that I am seeing (in BOLD what I believe is most relevant):
Name: Fizz
APN: mobile.bm
Proxy: Not set
Port: Not set
Username: Not set
Password: Not set
Server: Not set
MMSC: Not set
MMS proxy: Not set
MMS port: Not set
MCC: 302
MNC: 500
Authentication type: Not set
APN type: default
APN protocol: IPv4
APN roaming protocol: IPv4
Enable/disable APN: Enabled
Bearer: Unspecified
Mobile virtual network operator type: GID
Mobile virtual network operator value: 55

I guess so. Sorry, we couldn't help you.