At the beginning of Thor Ragnarok Heimdall is not seen as the Bifrost keeper, but was given the all seeing power by Odin. But in the movie Odin was not present at Asgard. So he couldn't give the power to the new keeper. So how can a keeper who can't see everything guard the Bifrost?
Heimdell has the power of seeing, but Skurge didn't. So, they used some sort of communicator to listen Thor from other realms. When Thor asks to bring him back to Asgard after getting the crown of Surtur, we see his voice coming from some sort of radio device at Asgard. I can't get the picture at the moment, though.
Although we don't know how this device worked and we may never after the Asgard is destroyed.