Multiple servers with fallback. Either a proper failover setup, or just use DNS.

It learns a bit as you type. But if you want it to forcibly recognize a word go to settings > general > keyboard > text replacement and add the words it won’t recognize. You can even add a shortcut for common typos. But it will hard override any “well you touched the edge of this key but I think you actually meant this” so use the shortcuts carefully.

What did you do to your phone? I’ve never had any issues with mine trying to autocorrect die.