Yeah...this isn't a problem I've had, at least not regularly. Once in a blue moon I'll try to change networks and it will fail to connect to LTE (it's always the wi-fi -> LTE transition that's the problem in my case, never the other way around). What I do is turn airplane mode on and off, and that fixes the problem.
Mind you, this is quite rare for me. In four months with the S8+, it's happened to me four or five times, and I'd estimate that I make 15-20 transitions between wi-fi and LTE per day. It's happened with similar frequency to all previous S-series and Note-series phones I've owned.