Hey DavidJunior,
I see that you have a question about iMessages and your cellular data plan. I have an article that can provide more information about this subject, and it is found below:
You can temporarily turn off cellular data to prevent apps from using the cellular network to send or receive data. Here's what happens when you turn off cellular data:
- You won't see the cellular data icons in the status bar (for example, LTE or 3G).
- You won't be able to send or receive MMS messages. You can still send and receive SMS and iMessages (text or multimedia) when connected to a Wi-Fi network.
Learn about cellular data settings and usage on your iPhone and iPad (Cellular Model) - Apple Support
https://support.apple.com/en-is/HT201299
As you can see, your iMessages will be available when connected to a Wi-Fi network if cellular data is turned off. When cellular data is turned on, iMessages go through cellular data. To see how much data Messages uses on your cellular data plan, go to the page referenced in the following link:
If Cellular Data is off, all data services use only Wi-Fi—including email, web browsing, push notifications, and other services. If Cellular Data is on, carrier charges may apply. For example, using certain features and services that transfer data, such as Siri and Messages, could result in charges to your data plan.
Set whether cellular data is used for apps and services. Go to Settings > Cellular, then turn cellular data on or off for any app that can use cellular data. If a setting is off, iPhone uses only Wi-Fi for that service. The iTunes setting includes both iTunes Match and automatic downloads from the iTunes Store and the App Store.
Cellular settings - iPhone
http://help.apple.com/iphone/8/#/iph3dd5f213
You can find the relevant information under System Services>Messaging Services.
Thanks for being a part of the Apple Support Communities!
Regards,
Braden