In the US you are going to have to use LTE. I usually recommend turning LTE off to prevent data leakage even when mobile data and roaming data are turned off on the phone. But turning off LTE or VoLTE in the US will leave you without a working phone.
So you may end up with an EasyRoam charge here or there for a Kilobyte or two of data usage. Usually Koodo will refund these without much issue here on the forum or in a callback.