Why would the bike need 4G? The bike doesn't have mobile services, only the phone. The firmware update is applied to the phone and then you apply the update from the phone to the bike.
Again, that's how it works now apparently, but up to some months ago it was the bike that needed to be brought to a place with 4G reception, since at least the gen 1 up to some time ago did have mobile service, and the phone had 4G connectivity, too. Back then, this topic was discussed multiple times.
Watch that video tutorial I've posted, they didn't mention doing anything with the phone.
Now I don't know whether the 4G connectivity of the bikes has been disabled and only the phone is connected, but if back then, with those older firmwares, it wasn't possible to update using the phone, then I suspect that you can't do that even if the app is updated because both the phone and the bike need to have a recent enough update.
So, unless for some practical reasons it's not possible to keep the bike somewhere with good 4G reception, I would definetly give it a try. I remember that the bike needed to be exposed to 4G for quite some time; I remember not receiving the update I was looking for even like after half an hour of 4G exposure and then going to the track some time later and finding out that an update was available.