@Paris can you try using a different version of libp2p mplex? npm i libp2p-mplex@0.8.5. There may be an npm version tagging issue we need to correct that could be causing your problem.
@shahafa is your code up anywhere that I could take a look at?
We are close to releasing a large refactor of js-libp2p and will be updating the examples on the docs site to match. As part of this I would like to get the full example code linked to a repo so that it’s easier to verify against what you’ve built. Would that be helpful?