CANNOT SEND GROUP TEXTS CONTAINING ANDROID USERS!

I had this same issue and was about to go to ATT and switch until I checked this article. Didn’t have to reset network settings, but I did insert a google fi sim, clear the network settings under ‘cellular’ and was able to text again. I loved GoogleFi and would have stayed, but ATT has better reception in the house we are moving to. Hope this helps someone going through the hours of visiting the att store / being escalated to tier 2 tech support. 

Popping the Google Fi Sim back in and clearing the Settings definitely fixed the issue.  I've been stuck with this issue for the whole year and I forgot I had inputed a proxy in the SMS settings to make it work for the country I went too.  I couldn't see the settings with the ATT SIM card in, but I could definitely see them with the Google Fi SIM card once I put it back in.   Once I cleared the settings and turned off the phone and swapped the ATT SIM card back in everything worked as expected again.

I'll give my update on this since I just figured it out with the help of many others who posted solutions here. (I actually joined this forum just to do a write up because it is confusing and can save others hours of effort). I spent several hours over many months dealing with this; many hours with AT&T corporate technical support & in their retail stores trying out all combinations.

The big 'ah-ha' moment for me was realizing that the Google Fi (and maybe other mvno options) Cellular Data Network settings (i.e. APN & MMSC) used for their plans are stuck to that type of SIM (physical or eSIM) on the iPhone. When you switch to using an AT&T SIM, the ability to change these settings is 'hidden' by AT&T but the actual values set for Google FI (or MVNOs) still remains set/active on your iPhone. THIS IS WHAT BREAKS MMS/SMS.

Second 'ah-ha', it actually is possible to operate in Dual-SIM mode on iPhone with AT&T and Google Fi and have 100% functioning MMS/SMS.... but holy moses the steps are very specific.

Here's my final setup because the type of SIM for the specific carrier (and their settings) matter in my example below.

• AT&T on Physical SIM
• Google Fi on eSIM (but I have the Physical SIM and SIM code still - this is a MUST)
• iPhone 12 Pro (iOS 14.4)

So adding Google Fi always starts with the physical SIM activation. In doing so, their instructions say to configure the settings in Settings > Cellular > Cellular Data Network. (This is 'hidden' when using an AT&T SIM (either physical or eSIM). These are the settings that cause issues with AT&T SMS/MMS.

1. My phone starts out life with only one Cellular plan, AT&T on a physical SIM... no problems so far. 
2. I add Google Fi service, requiring me to swap my AT&T SIM card out, and put in my Google Fi SIM. (iOS will still have the AT&T cellular plan but it will show with "No SIM" under Settings > Cellular when you pull the AT&T SIM card out)
3. I download and launch the Google Fi iOS app to start activation
4. The instructions say to update my APN + MMSC settings in Settings > Cellular > Cellular Data
5. Everything activates, I reboot my phone.
6. *In that same menu you change settings in Step 4 above, you also have "Reset Settings" <-- make note of this for later

I want to run in Dual SIM and the most efficient, self-service, method I have available at home is to make Google Fi eSIM. (AT&T's process is more complicated and requires them to help you)

To get Google Fi to eSIM, follow their published online instructions. They're basically as follows:

1. Make sure your physical SIM is inserted. (It would be if you continued from above)
2. Delete and reinstall the Google Fi iOS app.
3. Sign in to the app with your Fi account.
4. When prompted, tap Download your SIM.
5. Then add the cellular plan with QR code as instructed. (I recommend you label this plan with something you will easily identify this is the eSIM version. It will help later when you need to distinguish which Google Fi plan was for the physical SIM once you've changed to eSIM)
6. Return to the Google Fi iOS app, finish the steps, and power off your phone when they say you need to reboot.

So you reboot your phone - but your instinct may be to remove the now unneeded physical Google Fi SIM and put your AT&T SIM back in the tray. If you do this, those Cellular Data Network settings you first entered will still be set AGAINST your physical SIM slot. Using the AT&T SIM card will hide your ability to get back to this Cellular Data Network menu and reset these settings on that SIM slot - thus your SMS/MMS issue will persist when the AT&T physical SIM is used.

Turn on your phone WITH the Google FI physical SIM still inserted. You can check the Google Fi app under 'PLAN' > Manage plan > Devices > iPhone##,# , and it should show you 'Google Fi eSIM' to confirm. You should also have working service from Google Fi.

• Go back to Settings > Cellular > Under 'Cellular Plans': Choose the plan for the physical Google Fi SIM (NOT THE eSIM) > Cellular Data Network > Reset Settings.
• Now you can back out to Carrier (You can choose to Remove Cellular Plan, or not)
• Eject the Google Fi SIM.
• Replace the AT&T SIM
• SMS/MMS on AT&T should work as normal (I didn't need to reboot or toggle cellular radios) 
• Both AT&T and Google Fi service should be working together normally

If you find a left over cellular plan in Settings > Cellular that says "No SIM", you can ignore it or remove it... 

These steps, especially for Google Fi stuff, could change later but they are accurate currently.

Just as JamWils posted earlier: You can perform this settings reset at anytime if you first swap your active AT&T physical SIM out, and put your Google Fi SIM back in (even if you're using Google Fi as eSIM). 

I hope this helps - it seems nobody at AT&T really understands the underlying issue here. I promise you do not need to reset any other network settings, or iPhone factory resets to get this to work.