Thanks jecapaga.
This is my same experience. Spent 2 hours last Saturday with Apple, installed profiles on my XS Max and iPad Air 2 for comparison. Ran speedtests on wifi and cellular. Created system diagonose files and sent those along with screenshots to Apple. They followed up with me again this past Monday to spend another hour with me. They are sharing all my logs and files with engineers; different teams are handling the LTE and wifi issues.
I am glad to be able to help, but it has been frustrating spending so much of my personal time trying to get them information. They started asking me to do the same with my wife's iPhone 6S, but I had to draw a line somewhere. Firstly my wife is really sensitive about privacy and didn't want to share information and also I quite frankly didn't want to spend another few hours going through the process all over again. I work and getting on calls with Apple at 8pm after a long day is too much.
I was told I would likely be contacted later in the week.