My workaround for the same problem ...
Google for "NEO Coolcam Z-wave plus plug range upgrade"
Works OK for me, as the NEO Coolcam power plug is located inside the kitchen under sink storage, so usually no one reaches there and I consider the mod relatively safe.
There are three brick walls between my C7 and the Neo Coolcam plug and after the modification it works without problems ( no drop offs, no noticeable delays ) :
