Green Bubbles

My dad has an iPhone XS, and has the latest sofware update as of April 14, 2024. When I send him an iMessage, his bubbles appear green. His phone has recently developed to where it can no longer send photos to anyone, and all the contacts on his phone when sending texts appear as green bubbles, regardless if iOS or Android. I went to the messages on the settings app, and it only has the "Turn on SMS" toggle but nothing for the "Turn on MMS" toggle. I went to cellular and it does not show "messages" as an app that uses cellular data, no toggle feature. I ordered a new sim card, this time an eSIM, but the same issue prevails. It is not a phone issue, as I had the phone he currently has now, and I never encountered that issue. The carrier for our family is MINT mobile, Ryan Renalds. Thanks!!