I tried using the stock samsung messaging app and also google messenger and again no prevail. it's not just messaging though, it is wifi calling as well. and yes wifi calling is enabled.
Sometimes it does like when I initially turn on wifi. Then after a few minutes I lose all functionality of wifi calling and messaging. but still shows strong wifi connection and speeds.
Since you can't add another apn you're going to have to deal with spectrum or if your friendly with the networking people at your data center ask them to do some packet captures of your phone to see if they're seeing anything unusual in your connection between your phone and Verizon. If and when you talk to spectrum ask them if your phone is provisioned for 5g and that you have the correct apn settings escalate possible
Okay I will call spectrum again. I know my phone is provisioned for 5G as I had to call and get a new sim card for the 5G. I will try all that here shortly and let you know what they say.