Hello Tingers: My iPhone Xr with newly ported number (from Spectrum VOIP) fails to send group SMS messages if any of the recipients are SMS-only (i.e., Android). Sending group messages to a group of iPhone users works fine, and sending individual SMS messages to the same recipients that fail in a group SMS also works fine.
I've verified that Settings->"Send as SMS", "MMS Messaging", and "Group Messaging" are all enabled. Using other iPhones (and numbers) on the same account works fine; it's just this iPhone Xr with the X3 SIM that I set up a couple days ago that's having the problem.
Can you check settings on your end to verify that MMS has been enabled for my newly ported number?