Ya, I'm in the same boat with AC 4 (pun intended). I've done a lot of searching for solutions, most of which have been a few years old, but rightfully so.
Anyway, I've uninstalled and reinstalled the app several times now. Tried turning the app on prior to the game and then turning the game on. Ensured "all" of my software was up to date (on both ends *mobile and console*).
Heck, I've even tried force feeding the sync. By that, I mean I wanted to see if it was my phone, console or the app. So, I tried to sync my phone to my console and, well, that worked...
Sadly the app sync still didn't, so that's telling me that if my phone can successfully sync to my console in multiple different apps and be used in them, but then fail to sync in the companion app, that there's a glitch in the app.
Now the glitch could be with the software in my phone as well or some unknown. But, going to try to send in a ticket and see if they can help.
Phone: Moto Z Force Droid (Android)
Console: PS4 (Fairly new, store bought Nov. 2016).
last 3 days ago i try and all connected , i think the server only hold limited user maybe maximum 100 people using it , but i made big mistake by closing my console than android which make it log out from server. this time if i able to connect again i will log off from phone first not from console .2 days i still connected with p
android my last location and so on, maybe only limited user