This is not specific to Samsung Galaxy S9+ but I don't know where to start. A few days ago I lost the ability to text someone. We can call each other. I can get his texts but when I reply, he never gets my messages. We've both been through our phone settings. We're not blocked as far as we can tell.
He's about to get a new phone with Verizon (was AT&T). Maybe this problem will go away. But if the problem is with my Galaxy S9+ then it could still happen.
Yes I have texted other people and I can send and receive as expected.