I found out how to use Xcode 9.1.4 to install to a device running iOS 12.1 by following the instructions in this article.
Basically, one installs the latest Xcode beta, then opens the applications file by right clicking, select the Xcode-Beta directory, right click on it to show the contents, move to the Developer->Platforms->iPhoneOS.Platform->Devicesupport.
Copy Device support.
Do the same with the Xcoide9.1.4. application directory and paste the Devicesupport to Developer->Platforms->iPhoneOS.Platform
Alternatively, one could set up a symbolic link, but I haven’t tried that.
Hope this helps someone else with a similar problem