Had this same issue. I bought my phone through Verizon on release day, but I went to Apple and told them I experienced the issue when the phone was brand new, and when using a SIM from another phone. Almost no question, they replaced it immediately. So far, the drive home was locked in LTE, like always with my 6S. Will let you know if it improves after the drive to work Monday.
I wish I took pictures, but I noticed the refurbished or replacement phone Apple gave me had noticeably darker antennae lines compared to my day one iPhone 7 Silver 128GB which had antenna lines which were brighter white. Probably nothing, but would be interested to see comparisons.
Never had this happen, maybe its new, but the Apple rep said my phone was going to be delivered to engineers, and asked if they had my permission to contact me and ask me specifics of the issues. Either that is new, or they know something is wrong.