When you send a message to another contact using an Apple device, your iPhone sends it via iMessage by default. If your iPhone isn't sending iMessages and MMS, make sure you're connected to the internet via mobile data or Wi-Fi. It's a good idea to make sure your iPhone can send messages as normal text messages when iMessage isn't available. Check in your iPhone's Settings app if various messaging options are turned on so that your iPhone can send messages when iMessage fails. 

Check also - https://emailspedia.com/iphone-is-not-sending-texts-to-android/