T-Mobile user here. I spent a week out at Elfin Cove travelling to/from Juneau via boat. As stated above, no issues with 5G in most of Juneau proper via GCI roaming with some dead spots as one drives north past Auke Bay to the end of the road. Heading out to sea, surprisingly good LTE coverage for about 40 miles and then it becomes very spotty with mostly AT&T service. In Elfin Cove proper, the only reliable connection limited to text messages.