Officially
"Files app (iOS)—Copy the basemap file (.tpk, .tpkx, or .vtpk) you want on your device to a website that is available in the Files app, for example, iCloud Drive, Box, or Google Drive. In the Files app, browse to where you uploaded the basemap file and copy it to your device, placing it in the Field Maps folder. See Apple Support for complete steps on using the Files app on your device."
should work - source https://doc.arcgis.com/en/field-maps/ios/use-maps/configure-field-maps.htm or would. You could also contact ESRIEsri Support to assist.
