If you just need to access your NAS from outside your home network so your phone can syncthing to it, you can use syncthing in global mode -- it will use public

If you just need to access your NAS from outside your home network so your phone can syncthing to it, you can use syncthing in global mode -- it will use public relay servers to sync between phone and NAS. But maybe you need more than that / knew this already.