Nostr doesn't work like Twitter. When you post the note goes to multiple relays (servers). Those servers can never guarantee your note will be kept forver. Or when they go offline your note is offline with them.

A good practice I believe is to have your own relay. It doesn't have to be public but it will save your data (notes, followers...).

See nostrapps.com there's an app called Citrine which will spin up a relay on your phone (android). Then take the url from there and add it to the list of your relays of your client.

Another option is to host on your personal hardware with start9 - which could be a bit more complicated.