Hey, changing the server difficulty can be painful sometimes, so I would highly recommend using a plugin such as Multiverse.
Multiverse is a plugin which has a feature to force set the difficult of your world.
In-case you do not know how to install this plugin, go to your server Dashboard -> Edit Server -> Plugins and then type in "Multiverse" in the search bar. Install the plugin and Restart your server.
After that's done, this command will allow you to change the difficulty of your world: /mv modify set difficulty (difficulty-level) (world-name).
In-case you do not know names of worlds on your server, use /worlds to find out the list of worlds that are loaded on your server.
Here are a few examples for using this command:
-> /mv modify set difficulty easy world_nether.
-> /mv modify set difficulty hard world.
That's not it, Multiverse has tonnes of other essential features. If you wish to see them, check this official wiki: https://github.com/Multiverse/Multiverse-Core/wiki/Command-Reference