This is a bug affecting Collector, Field Maps, and Explorer when an Android is in a rural area.
BUG-000135548 When logging in to ArcGIS Collector or ArcGIS Field Maps (Android), an error message, "Untrusted Host" is returned.
This issue is fixed on the 21.2 version of the products. As Collector and Field Maps are on the 20.+ versions, you will have to wait until the next release cycle of updates. Unfortunately, I do not have a time frame for the update cycles.