According to Sprint’s press release, “When this rolls out, Google Fi customers with Sprint 5G compatible Designed for Fi phones in a 5G coverage area will be automatically connected to the network, quite similar to today’s seamless transitions between cellular networks, depending on the location.”
As Sprint notes, this will also depend on your location because how Google Fi works is by hopping onto the carrier with the best signal, so if you’re in an area where you’re having trouble hooking onto Sprint’s network, then you won’t be able to take full advantage of it. Also it will be dependent on you owning a 5G smartphone that is also “Designed for Fi”.
It looks like there will be a lot of caveats to truly take advantage of Google Fi’s 5G capabilities in 2019, but this is to be expected. We imagine that it will probably take a year or two before 5G becomes as ubiquitous as 4G, so unless you absolutely have to have it, maybe waiting it out could be a good idea.