If you have a mobile plan that includes free Messenger data, you can use the following steps to take advantage of it on your Android device:
Check with your mobile carrier to confirm that your plan includes free Messenger data. If it does, make sure that your account is up to date and that you have activated the feature.
Download and install the Messenger app on your Android device if you haven't already done so. You can find it on the Google Play Store.
Open the Messenger app and log in with your Facebook account. You should be able to use the app normally without being charged for data usage, as long as you are not accessing any external websites or other apps that require an internet connection.
To ensure that you are not using any data outside of Messenger, you can turn off mobile data for other apps on your device. To do this, go to the "Settings" app on your Android device, select "Apps & notifications," and then choose each app that you want to restrict data usage for.
Another option is to use the Messenger Lite app, which is a lightweight version of Messenger that uses less data and is optimized for low-end devices. You can find Messenger Lite on the Google Play Store and use it in the same way as the regular Messenger app.
Keep in mind that free Messenger data may be subject to certain restrictions and limitations, such as speed caps or data usage caps. Make sure to read the terms and conditions of your mobile plan carefully, and monitor your data usage to avoid any unexpected charges.