Exactly what "Samba app" did you install on your phone? If I assume correctly and you want to access video files, I would strongly recommend VLC Player.
That being said, I had irregular but more or less continuous problems of a similar nature while using the RT-AC86U (for just over a year). Once in a while I would completely lose wi-fi connection to the NAS, but sometimes LAN as well. On some occasions everything would come back to normal of its own accord, on others I would have to reset the router.
Some point to a Windows 10 update breaking the functionality, while others explain it by buggy Asus firmware - or a combination of both.
I never did get to the bottom of this, though, as my RT-AC86U finally broke down and I got a refund.
And one more thing - does ftp work (if of course you have it enabled)?