MMS No Longer Working in iOS 13.6.1 on iPad Air 2

I recently had to do a complete reset of my old iPad Air 2. I backed it up, did a reset which did a clean install of the latest iOS 13.6.1, then restored programs/data from the backup. Now I can no longer send MMS messages. This had been working fine for at least a couple years up through the previous iOS version 13.4 I had on this iPad. I have one old message thread that has 6 iOS users and 3 Android users. Now every time I try to send a message to that group, I get the error that the message failed to send and the red exclamation mark. Multiple attempts to resend keep getting the same result.

I have done searches on this topic and read dozens of threads here and other forums but just can't get this resolved. Some threads mention a MMS setting in the Settings app on the iPad, but I don't see that option in iOS 13.6.1.

Hopefully I'm missing something obvious!