The theory I read elsewhere is that we're witnessing a simulated representation of Bernard accessing Dolores' host-identity while her physical self is in a sleep-cycle, hence Bernard's constant assurance that she's "in a dream". Some have also suggested that there's a duplicate body that Bernard can summon her host-identity to in order to have these conversations.

Either seems like a somewhat plausible explanation for how Dolores could traverse what are presumably great distances.