Why did my iPhone Switch to MMS?

Thank you for your reply and screenshot: Re: "I figured out this was the culprit. How do I fix it?"

1. Go to the Settings app  on your iPhone.
2. Tap Apps, then tap Messages.
3. Turn on iMessage.
4. To select which of your phone numbers and email addresses you want to use with iMessage, tap Send & Receive, then choose from the available options below “You can receive iMessages to and reply from.”

(from Apple Support's article: Set up Messages on iPhone - Apple Support)

Re: Why did my iPhone Switch to MMS?

It suddenly switched to MMS messaging on a Thursday.

My iPad is my last Apple device to be using iMessage. Help me fix this.

Messages from Apple to Apple devices use wi-fi (iMessages)

yet messages to Android devices need to use cellular data (MMS).

View or change cellular data settings on iPhone - Apple Support

or

View or change cellular data settings on iPad (Wi-Fi + Cellular models) - Apple Support

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If you have Wi-fi Assist turned on, that can change messages from wi-fi (iMessages) to cellular data (MMS) if the wi-fi signal is inadequate.

About Wi-Fi Assist - Apple Support

Eg: says:

"Turn Wi-Fi Assist on or off

Wi-Fi Assist is on by default. If you don't want your iOS device to stay connected to the Internet when you have a poor Wi-Fi connection, you can disable Wi-Fi Assist. Go to Settings > Cellular or Settings > Mobile Data. Then scroll down and tap the slider for Wi-Fi Assist."

Apple Support's article: About iOS 26 Updates - Apple Support

says iOS 26.0.1 is the newest update:

"This update provides important bug fixes and security updates for your iPhone"