Switched SIM card back to new device and not working

Hi there @Jnobre21, we're here to help you with your iPhone 12 Pro Max SIM.

As stated by @sandblaster, have you been able to perform any troubleshooting steps for your iPhone 12 Pro Max?

• From the home screen, select the Settings app.
• Select Cellular, then select Add Cellular Plan.
• Use your iPhone's camera to scan the QR code. Select Enter Details Manually at the bottom of the screen to manually enter your plan information. 
• Select Add Cellular Plan.
• Label the Cellular Plans as desired, then select Continue.
  • Note: To label your plans, select the desired Phone number then select the desired Label or enter a Custom Label. Select Done when finished.
• Select your Default Line, then select Continue.
• Select the desired line to use for iMessage, Facetime and Apple ID, then select Continue. Select the desired lineto use for Cellular Data. If desired, select the Allow Cellular Data Switching switch, then select Done. 

Note: Both of your phone numbers can make and receive voice calls and send and receive messages. However, your iPhone can only use one cellular data network at a time. Turning on Cellular Data Switching will automatically switch lines for cellular data when you're on a call.

Let us know how this goes.

Ashley, AT&T Community Specialist 

What troubleshooting steps have you tried, any? Rebooting the phone and resetting network settings are the standard steps. You may also just need to wait awhile. Moving your sim from a 5G phone to a non 5G phone may have caused your line to be re-provisioned to 4G/LTE and not 5G. It may just take awhile for the system to re-provision your line back to 5G.