[Samsung Galaxy S9+ Questions & Answers] hyperblu:
The issue you're running into I don't think is necessarily firmware related. The phone is probably carrier locked to Sprint. Has sprint provided you an unlock code to switch carriers?

As far as switching firmware, because you're with an MVNO, personally I would not flash a "U" (carrier specific) firmware unless Boost has carrier specific features you would benefit from using (Which I doubt).

Reason I say that, Sprint uses a different technology/system to provide simultaneous voice and data and using a Sprint (or other U firmware) might cause compatibility issues, in that regard, because you're on an MVNO that can't/may not utilize Sprint's unique system.

That being said, Sprint has (finally) gotten on board with VoLTE which is universal, and has started to implement this into their network.

Most MVNO's have limitations on BYOP (bring your own phone) plans because of the firmware carrier restrictions placed on the phone. For instance, when I switched from Verizon to Straight Talk, I immediately wasn't able to use simultaneous voice/data because I was running Verizon configured firmware through a different carrier.

This was corrected by flashing a universal (Factory Unlocked) G965U1 firmware. This enables the universal VoLTE option the phone has instead of trying to bridge voice/data with Verizon's preset configurations.

Most MVNOs that sell devices with their service, almost unanimously, sell factory unlocked phones that aren't branded to any specific carrier.

Your best option, in my opinion, is to emulate the devices THEY sell (including Boost) by flashing G965U1 firmware (latest package is CSA8). Either download from XDA or Sammobile "XAA Unknown" region for carrier.

That firmware package, when loading it into ODIN you'll have two CSC files. Don't use the CSC_HOME file, use the other one. This will perform a full wipe (backup your data) and change your carrier to "Unknown" and effectively make your phone universal (which boost prefers).

Since Boost has already activated your IMEI on their network, your phone "should" automatically configure itself to their network at the initial setup after the flash.

If you continue to run into any hiccups, let me know and I'll try to help you work through them. Let me know either way how it works out for you. [emoji16]

---------- Post added at 06:42 PM ---------- Previous post was at 06:37 PM ----------

By the way, flashing U firmware will remove all traces of Sprint from the phone (All the bloatware) and enable free caller ID. Which is a plus of using Unlocked firmware. You can download the boost app through Google play to manage your account after you set up the phone again.

---------- Post added at 06:42 PM ---------- Previous post was at 06:42 PM ----------

U1 firmware, not U*

---------- Post added at 06:45 PM ---------- Previous post was at 06:42 PM ----------

I just noticed you said the phone was AT&T branded. As far as I know, AT&T Sim locks their phones and you'll need the unlock code from AT&T to use the phone on another carrier regardless of whether you use ODIN or not. I'm assuming you have this code if you're trying to use the phone dialer to change carriers, correct?