You have quite some problems with your code. I will list them below:

Required fixes

• As @about14sheep said, you must use await to get the actual response.
• You are not supposed to use JSON.parse() on a response, but rather response.json() to get the JSON body.
• Use of an incorrect operator, for numbers I'd suggest you use == than ===
• Before 10 seconds have passed, it will show undefined. To fix this, call the function before the setInterval().

Optional things I added

• Formatted JS code
• Added semicolons when necessary
• Used ES6 template literals feature
• Removed async when un-needed

I have fixed all of these problems in the code below.

let jogadores;

client.on('ready', async () => {

    async function players_online() {
        const response = await fetch(`http://ip:port/dynamic.json`);
        const data = await response.json();
        jogadores = data.clients;
    }

    function autoconnect() {
        if (jogadores == -1) {
            console.log('Offline');
        } else {
            console.log(`Online with ${jogadores}/5`);
        }
    }

    autoconnect();
    players_online();

    setInterval(() => {
        client.user.setStatus('dnd');
        client.user.setActivity(`Online with ${jogadores} players.`, {
            type: 'PLAYING'
        });
    }, 10000);

});