I recently switched from Verizon to Att and took my S& Edge with. I updated the Access Point Names. Internet is fine, I can call out, and SMS is fine. I am having trouble receiving MMS and some incoming calls. i can have good coverage, receive calls from some but not others. I dont know if I have some settings wrong or if its the Firstnet account. I welcome some help, thank-you all.
You need to talk to the carrier/service provider if they aren't working. They may need to alter some things on your account. Trying different codes to make it work. Codes they have that may be needed to make changes on the device. A friend of mine when through that when taking a phone from one company to another. The original company had altered it so that when switching to the new carrier/service provider she had to spend a couple hours on the phone with them getting all the different services to work correctly. So I would start there.