some text messages are not immediately received and delayed by hours up to days

I have been with T-Mobile for 17 years and in just the past 3 months I have had recurring text "black-out" periods, whereby my incoming text messages are being delayed for 3 days in each month. There are no words in the English language to describe how frustrating this is. After the "black-out" text messaging block is removed, I get a flood of text messages all at once. This is ridiculous! The most bizarre thing yet is the fact that I still get text messages from T-Mobile during the "black-out period" -- but no-one else. I demanded customer service let me speak to an engineer, and they finally agreed to get one on the line, after I expressed how close I was to cancelling my service with T-Mobile. The engineer believed he had a fix. He refreshed my phones OS which immediately removed the block, but it came back 2 weeks later. I think he knew how to remove the block and was gaslighting me about the problem being a missing software update. My job depends on me receiving text messages and I will have no choice but to dump T-Mobile if they can't give me back the service I have grown accustomed to for 17 years! In the meantime, I just want T-Mobile to tell me the truth. If they have lost or replaced their most talented technical people and are stuck with incompetent DEI hires, I can accept that and I might continue with them due to their honesty in trying to help me.