I'm also grandfathered in with an unlimited plan. I haven't any issues with my signal. Try going into settings, and go to mobile networks. Make sure it's set to CDMA/LTE instead of automatic. Let me know how that works.
I received the Note 8 Calling Plus update. I have to say it's very useful, but not all that reliable. Does anyone find it to be reliable on their end? Let me know please 😊