Some of this info has already been shared elsewhere but I ran into quite a few quirks when trying to get group messaging to work on the Dark Star network of US Mobile after switching from an iPhone. I figured listing everything I ran into here might save other people some trouble.
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The default APN is not editable, you have to make a new APN profile with the + button on the Access Point Names menu
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If your APN settings are wrong, no new profile will show up when you hit save
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Even though Dark Star is actually AT&T under the hood, US Mobile has their own APN settings - https://www.usmobile.com/apn-download?srsltid=AfmBOoo2GrmYjLsd3odaPY5WmtS7IY0OeZWKeYL3leAEGI9cbKeszicE
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Even if you’ve only had RCS on an iPhone previously, your phone may be registered with Google’s RCS service. It’s dependent on your carrier, not necessarily your hardware. In my case just disabling iMessage, RCS, and FaceTime wasn’t enough. https://messages.google.com/disable-chat
Once I got my APN settings correct and deregistered from Google’s RCS service, everything started working properly. The one thing that sucks is if you were in any group chats previously with RCS enabled, you won’t get any messages in them. From what I understand, RCS eligibility is only checked when group chats are first created. If you are no longer eligible to receive them, others’ messages in the group chat can still get sent as RCS.