Yes, unlocked and from Samsung. What explains the intermittent service then? Why would I have service sometimes and sometimes not if my service provider network settings have been incorrect since the software upgrade? Wouldn't they always be wrong and never let me connect to T-Mo until I updated them?
It's a possibility. Or it could be a bad download of the update. My experience I had a global unlocked galaxy a71 I got from my son. I had service with straight talk at the time and had the no Sim intermittent issue you have. I fixed it by researching the carriers apn for my area and put it into the apn settings tab. After that I had great service till the phone was replaced by my new boost mobile Samsung I have.
I also purchased my phone directly from samsung. I had a service representative turn on developer mode and say he fixed my phone -_____- I'm currently looking for new phone.
For your phone glitching and all that nastiness sometimes you have to go into settings and do a factory reset of the device to clear the left over install files and random data left from the older patch you had previously. I have had to do this on many devices I have personally owned over my lifetime and 99% of the time it was the correct thing to fix it.
You may have gotten a bad Ota update that got broken upon download. If you have a pc download samsunk kies or smart switch....whatever they call their rescue tool now and follow the directions to flash the latest update to your phone. This is the last thing you would ever have to do before sending it in.
Kies was the older rescue tool. It's Samsung Smart Switch now. It will allow you to upgrade the operating system on your phone to fix the botched update.
