iPhone No Service After iOS 12 Update

My question is why should I still be an iPhone user? After having many issues with the previous version of iOS 11, I updated to iOS 12 as soon as it was available. After the update, I've been have issued where the phone shows it's connected to the cell towers with full service and LTE. In actuality, I have no service or LTE at all. I would restart my phone then it would work for a few hours. This happened every day after the update. Then last week, it wouldn't even connect to the cell towers at all. I went to T-Mobile to check with them. They told me since my phone was old I should buy a new one because it was probably a hardware problem. I told them this was unacceptable. I had no issues like this prior to the update. THIS IS A SOFTWARE PROBLEM! Then I went to the Apple store. I told the employee I wanted to setup an appointment to see if they could run a diagnostic test on my phone. The employee told me sure, it will be in an hour and forty five minutes. I ask them if I could setup an appointment in advance instead. That way I wouldn't have to wait. Which he replied "Yes, but we are three days out." I told them to just get me in line and I'll have to wait around. After I finally had a tech to look at my phone. I asked him to run a diagnostic test which replied, "We can't do that but you can try to restore your phone." I told him my backup for the phone is the same version of iOS (12) that I've had problems with. Which he then told me since my phone is old, it is not worth repairing and I should purchase a new one. I told him this is unacceptable. I bought my phone from you, I updated my phone because I was having so many issues and now it’s even worse since it’s a paper weight thereafter. After I left I went online and read many other complaints about the same issue after updating to iOS 12. Even after 12.0.1, it is worse. Now I can't even use my WiFi to text people who aren't iPhone users. I know this is a software problem, not hardware. If it’s a hardware problem, then the update somehow destroyed the hardware inside my phone. This doesn't make any sense if you know anything about hardware and software. I'm wondering what I should do next? T-Mobile won’t help me and neither will Apple. Would you remain an Apple user? I have been a longtime user of Apple products but this may be the last iPhone and/or Apple product I ever use.