After having my new iPhone 11 for only 3 months, I have had to send my phone in for repair with boomerang. I sent my phone in on the 27th of December 2019 and just got it back on the 16th of January 2020.
The issue was that my phone doesn't register reception in high reception places, If I place the same SIM card in my spare phone, in the same location, the phone will register the reception and internet. I have factory reset, reset my network settings and turned the phone of hundreds of times. I sent the phone away for repair, it came back with them saying that there was nothing wrong with the phone, yet I am still using my back up phone, which hardly works on its own, but somehow works better than my NEW iPhone 11, which is just as broken as it was when sent away.
Im paying for a new phone that doesn't even work, and I am sick of it!
That does sound incredibly frustrating. I wonder how the Repair Centre tests for reception issues, if it picks up reception and their location I'm not sure what else they would do.
Perhaps you could try your luck with Apple Support? They're generally pretty helpful.
Let us know how you go.