Have you tried resetting the network settings on your phone?
Have you tried clearing the cache partition on your phone?
Brand new Galaxy s20 FE with AT&T.
I tried all the solutions on here yet none seemed to work. Switched to the unmetered on my WiFi, etc. If I disconnect my WiFi on my phone, the text messages will come through. But, it's a pain to keep doing that.
I called Samsung and they told me to clear my cache, same as the originaly solution. I did that already! But, I only did that on my one messaging app. What they meant was to clear the entire cache by going into the developer mode on the phone. To access that, you need to turn off the phone and then hold the power button + the up volume button. Use the volume keys to scroll and the power button to select to clear the cache. It takes a second. Then reboot the phone.
I did this recently and so far it's working well. Fingers crossed it actually fixed the problem!