Suddenly not receiving text message with 2nd stage authentication from other apps.

Hi sgbiii,

If you are not receiving third party authentication codes, quit the Messages app and restart your iPhone which can help:

1. "From the Home Screen, swipe up from the bottom of the screen and pause in the middle of the screen.
2. Swipe right or left to find the app that you want to close. 
3. Swipe up on the app's preview to close the app. 

"

Close an app on your iPhone or iPod touch - Apple Support

1. "Press and hold either volume button and the side button until the power-off slider appears.
2. 
3. Drag the slider, then wait 30 seconds for your device to turn off. If your device is frozen or unresponsive, force restart your device.
4. To turn your device back on, press and hold the side button (on the right side of your iPhone) until you see the Apple logo.

"

Restart your iPhone - Apple Support

If the problem persists, we'd recommend reaching out to support for some additional options. Contact Apple for support and service

Cheers.