Cache message discord.js - node.js

Question

I would like to make some reaction roles. But for that, I have to cache messages which were sent before the bot started. I tried it with channel.messages.fetch, but that hasn't worked so far.
My current code:
client.on('messageReactionAdd', async(reaction, user) => {
client.channels.cache.get("689034237672030230");
channel.messages.fetch('708428887612194979');
// When we receive a reaction we check if the reaction is partial or not
if (reaction.partial) {
// If the message this reaction belongs to was removed the fetching might result in an API error, which we need to handle
try {
await message.reaction.fetch();
} catch (error) {
console.log('Something went wrong when fetching the message: ', error);
// Return as `reaction.message.author` may be undefined/null
return;
}
}
// Now the message has been cached and is fully available
console.log(`${reaction.message.author}'s message "${reaction.message.id}" gained a reaction!`);
// The reaction is now also fully available and the properties will be reflected accurately:
console.log(`${reaction.count} user(s) have given the same reaction to this message!`);
});

Answer 1

What you have to do is first fetch the guild where the channel is in and then get the channel from the guild (so it returns a GuildChannel). Then fetch messages from there.
Full code would be:
client.on('ready', async () => {
const guild = await client.guilds.fetch('guild-id-here');
const channel = guild.channels.cache.get('channel-id-here');
const message = await channel.messages.fetch('message-id-here');
});
client.on('message', message => {
//rest of your code
});

Answer 2

channel.messages.fetch() is not working because channel is not defined. You need to define the first 2 lines as variables:
const channel = client.channels.cache.get("689034237672030230");
const msg = channel.messages.cache.get('708428887612194979');