iPhone unable to receive One Time Passcodes (OPTs) on iMessage

I am at my wit's end here. After having this exact some issues with my previous iPhone XR, I am continued to be plagued by not receiving OPTs.

A bit of context on what's happening. When I am logging into a number of different apps, I am prompted with two-factor authentication where the app will send a passcode to my mobile device. It doesn't matter then app I'm using, the issue is the same. I hit "send me the OTP" and nothing comes through to my iPhone. I've checked all the settings and suggestions on this forum to no avail. I also spent the last two hours on the line with AT&T advanced customer support, who were also unable to remedy the issue.

Some of the settings I've already checked:

• Setting > Messages > Send & Receive is turned on for my phone number
• Settings > Messages > Send as SMS turned On
• Settings > Messages > Filter Unknown Senders turned Off
• Settings > Cellular > Cellular Data turned On and Roaming turned Off