Yes, it has the same MMSC settings as the full Android. Sometimes I can send MMS messages but can’t receive replies.
Did you change the app setting to “send as MMS” for multiple recipients?
To clarify “lost incoming MMS replies” are they group MMS or individual MMS?
I’m losing group MMS replies. As of now, I can’t find the “send to MMS” for multiple recipients. I remember seeing this in the past and it was on.
Enable these: “Messages App > top right 3 dots > Settings > Send group messages as MMS (and send long messages as MMS)”
Thank you. I checked the MMS settings. Both send group messages and send long messages were already enabled.
So from your above comments, is it possible RCS is re-enabled for the account (when you put it back in a RCS-enabled phone for re-testing)? If so that could be part of the issue.
As another question, is it mainly iPhone / iMessage that are getting lost?
I don’t think that is the problem. I disabled google messaging (RCS) about a month or so prior on my old phone and by request and never got group messaging to work. When I switched back, I did get some lost group messages. I’m now using my Brax3/Iode using second phone number. For this second phone I still can’t send or receive group messages from iphone and imessage. 95% of my friends and family use imessage,
Have you contacted your carrier (not sure if on T-Mobile or AT&T) to see if there is anything to check on their side? Make sure to clarify your system does not support RCS even though they may identify your device as being RCS capable. In the past, without them giving me clarification as to exactly what they were doing I had my carrier (US Mobile - Verizon or T-Mobile SIMs) “make some changes” that solved some mysterious missing messages issues. Black magic for sure 
but it is worth contacting them if nothing else seems to sort this out.
Yes, I contacted them multiple times and have spent hours with them on chats and on the phone. Nothing worked, no black magic for me. I don’t remember if I mentioned that the device was not RCS capable. If I do try again I will be sure to mention this. My carrier is Mint Mobile, a discount version of T-Mobile. I don’t know if using a different carrier would help. For my main line I switch back to my Samsung that was broken and then to an iphone for the first time in three years. Of the three operating systems, I like IodeOS the most but I need to have group messaging working.
OK sorry I forgot you were on a Brax3, yes RCS isn’t supported on it.
Can you confirm what APN settings are entered from “System Settings > Network & internet > SIMs > (your sim) > Access Point Names”? Particularly pay attention to the “MMSC” field as that is the key to MMS.
Note I also have a Brax3 for testing with a US Mobile (T-Mobile) SIM, but am using the default T-Mobile APN, so this:
NAME: T-Mobile US (can be set to anything only for display)
APN: fast.t-mobile.com
MMSC: http://mms.msg.eng.t-mobile.com/mms/wapenc
Testing before (with Ubuntu Touch some years back now) showed me that for these T-Mobile resellers that you can typically “mix and match” between the main T-Mobile settings and the MVNO settings, both will work. So you could try changing to the above to see if there is a difference. Or if the above is what you already have, then you can set to the Mint APN and MMSC instead (this is from my notes, possibly it is outdated?):
Name: Mint Mobile
APN: wholesale
MMSC: http://wholesale.mmsmvno.com/mms/wapenc
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