Try these instructios, whilst I understand it's for the S8 it's very similar as it's Android based.
I know this was posted a long time ago but I just had the same issue. I went into settings and activated Gboard and then went into manage keyboards and turned off Voice keyboard and it automatically went to physical keyboard. Without Gboard active I couldn't turn off Voice keyboard or activate Physical keyboard,Voice keyboard would not turn off. Drove me crazy and only by pure luck it worked. I only had this issue after letting the phone do an update.
Thanks very much for taking the time to post your own experience with this issue, Debbieg_. I'm sure our members who encounter this will find it most helpful.