I am continuously disconnected randomnly from the host without a reason... I don't think it's a problem of my connection, I use a fiber and the signal is always good. I have the Nat open so I also exclude that ...
The game already isn't amazingly optimized for switch, keep this in mind because the handheld and docked experiences can be different.
The best Answers that I can give you is that it could be a connection issue to the Switch online service.
If it's not this in particular, and you experience lag related issues, it could very well be a connection issue on your end after all. Is your router/modem up to par with fiber? Is your ISP(Internet Service Provider) limiting your connections in some way? Is your router prioritizing your other devices ie. 2/3G vs 5G?
Once again, the game is still fairly new to being ported to the Nintendo Switch, so optimization and stability isn't the best for the time being. I'm sure it will improve over time.
Hope this helps.