I have a theory as to what could be causing this issue as it is happening on my 6 plus too, all the hardware seems perfect and all software fixes have been tried.
I have noticed that the loud speaker module is acting more magnetised than normal at the bottom of the phone so much so that the magnet can pick up objects through the screen, something which I had previously not noticed.
With a strong enough magnetic force in the presence of the cellular antenna hardware connection at the top of the loudspeaker connection this could be causing signal interference on the cell antenna.
I do not have parts currently to test to see if this is in fact the cause unfortunately but this could be the cause.