Not flying one segment of ticket

Buy a new ticket?

You understand correctly and as far as I know there is no 'proper' way to skip that segment without consequences.

If you bought the ticket from an American airline then you will have a credit for what you paid. You can first try changing just the return portion (effectively changing from a round trip to a multi segment trip.) If that is not possible, you may need to cancel and rebook

If you purchased your ticket through the airline, then go to the website and change your outward journey to Amsterdam. You can change the destination (I just did it through AA on the main cabin tickets). It will tell you if there is a price difference. it may be more or maybe less. (if less you can get a credit). You won't be able to do this if you booked a basic ticket. You could also cancel the whole flight for an e-credit (If ticket class allows), and then simply book what you want. However, costs may have gone up since you bought the tickets and you will have to pay a difference.

Don't skip the Amsterdam to Brussels leg of the journey otherwise. What you are doing is skiplagging, and not realizing it. The airline will cancel all of your remaining flights once you miss any leg of your trip. You have to do the journey in sequence as outlined on your ticket.

There’s been quite a bit about this in the news lately with the warning that airlines are clamping down on people doing this. It would probably be smarter to just rebook your ticket.

Or, skip Rotterdam and go with the original plan.

The only way to achieve what you want is to deal directly with the airline. In a situation such as this, it will be easier, and everyone will have a clear understanding about what you want to do, if you speak directly with a real, live, airline person. Call them and see what can be worked out.

Just to confirm - I just had to do with through United. I had an initial segment and an ending segment that, due to my child’s new job and move, we no longer needed. However I called the airline to get it done. After I return, I can call and receive some sort of credit for those dropped segments. But yes, they need to know and redo your tickets.

Thank you for everyone's reply and recommendations. I will call the airline directly. Have a wonderful weekend!

It is a bit odd to fly from Amsterdam to Brussels with a layover of 4 hours, when the train takes less than that though. Many airlines would have put you on a train for that, and skipping the train (or getting off in Rotterdam where all trains to Brussels stop) would not have been an issue.

I will call the airline directly.

So what happened? Which airline?