Hi @Vilink welcome to the forums,
As @ArmchairTuna66 mentioned, the starting island is in the centre of the map. This is always the case for saves. If you’re playing multiplayer it may be tricky to navigate back there, depending on how far you traveled. But if you are playing a single player save, if you open the cartographer the island highlighted with the red square is the one you’re on, once the island in the centre is highlighted in red you’ll know you’re back on your first island. (Multiplayer does not affect the cartographer in the same way, this will always highlight the middle island even if you’re not there)
No workarounds have been successful so far if the trophies are already crafted, unfortunately. The team are aware of this issue and will be working to resolve it however I do not know how long that will take / when a fix will be released.
I have a Known Issues post on this here: [Known Issue][PS4/XB1] "Call Me Ahab" Trophy not unlocking for players and if anything changes I’ll update that post.