I just switched from Mint Mobile, since they don’t support wearable devices. I have the same problem sending multi-media SMS texts - iMessages via wifi and basic sms texts send without issue. The workaround that I have found is to cycle airplane mode thereby forcing reconnect and reprioritization of your phone connection on the current network. Multimedia texts will now send quickly without issue or delay.
However this is fleeting and has to be cycled anytime I want to send a SMS text with a photo or video attached.
me: iphone 12, ios 15.1, visible unlimited plan