FWIW, I have an SM-G965U1 stock unlocked I bought from B&H. I think the last OTA update I got was April with the January security patch. However, I went into the Samsung Experience store while visiting my brother last month to complain. Without blinking an eye they said they'd be happy to update my software. About 30 minutes later I'm now running G965U1UES6CSH1/G965U1OYM6CSH1, Security Patch August 1, 2019.
If you live anywhere near a Samsung Experience store you can schedule an appointment online and be in and out of the store within 30 minutes. They seemed to indicate that there was a problem with OTA updates after the update to Android 9. However, they believe my phone will now update OTA when new software is released. If not, I'll just check whether there's a Samsung Experience store that's convenient whenever I travel.
I'm going to type the same thing again,It is not Samsung's fault it is the cariers,I had the same issue and I was on Tmobile with my Galaxy s9 plus unlocked ,so I inserted different sim card (Mint mobile) that was not even activated !!!!! Guess what when I checked for updated thru the wifi I got 2 updates in a row!!!! Now I'm on the September security patch