Hello Raphael

Since you are talking about a very slow internet connection, I assume that your connection to the internet takes only a fraction of the power of the modem / switch. The NAS will hardly affect your idle internet connection. Even if you exchange files between the NAS and the computer, since your Internet connection is poor, you will probably not notice any difference.

You can also connect the NAS to the modem without hesitation (if it is not a stone-age model).
The traffic is only internal and does not restrict your line to the outside.

With newer routers you can also prioritise certain network traffic so that, for example, the internet connection always has priority over the NAS connection.

However, if you access the data of the NAS from outside, this will logically affect your internet speed. Here, the upload provided by the provider is especially important, as the files have to be "uploaded" by you to the internet.

Connect... The big problem is that I have a very very slow internet connection and therefore I don't want to connect the NAS to the modem!

So if I have understood this correctly, I could load the data from my PC via the switch to my NAS without the data having to flow via the modem/router?

I assume that during such a data transfer I can't access the internet or that both simply take much longer...

Would this version have any other disadvantages?
Which ones?

Can I have the same internet speed during normal use as without a connection via the switch?
For example, when the NAS is off?
Or vice versa, when the PC is not running and I want to access the data... Online?

Many thanks in advance

Raphael