Conventional trains

Anything but a Thalys or ICE.
As "normal" trains in Belgium and the Netherlands do not have reservations, you have no seat numbers assigned and you can sit anywhere that is free.
A suitable train would be (for example):

Bruxelles Midi depart 09:50, Antwerpen Centraal arrive 10:35
Antwerpen Centraal depart 10:45, Rotterdam Centraal arrive 11:56
Rotterdam Centraal depart 12:09, Amsterdam Centraal arrive 12:51

Note this involves two changes and takes 3 hours. a Thalys would be direct (no changes) and take 1h50. I hope the saving was worth the time and inconvenience.

I sent an email requiring about it and they said an IC train would be fine and that it was direct

"Anything but a Thalys or ICE."

There are no ICEs between Brussels and Amsterdam. ICE is a German train. The only way you could go by ICE would be via Köln, and that would not be the route of your ticket.

Although Thalys is owned by France, Belgium, Netherlands, and Germany, it is considered a private company with premium trains, so normal tickets are not valid on Thalys.

Even in Germany, if you buy a ticket valid on IC/EC, you would need an upgrade to use it on an ICE.

Where did you actually buy the tickets? Per your other post,

The website also says the following:
Special information: Ticket valid for any conventional train on the requested route during the whole validity period of the ticket (for homeprinted tickets, the validity period is limited to the requested travel date). Not valid on high speed trains.

Yes, so you can stop off in Antwerp for a few hours and take a later train, as long as you get to Amsterdam on the date of the ticket.

I bought my tickets through SNCB Europe. They say there are direct IC trains from Belgium to Amsterdam. If I can just hop on without a seat reservation that's fine.

SNCB-Europe is a ticketing agency and a subsidiary of the Belgian National Railway. You are the first person I've heard from who has used them. They sell tickets for Dutch, Belgian and French trains, plus Eurostar, Thalys, and German ICE trains. https://www.b-europe.com/Travel

I see the direct IC trains now on the Deutsche Bahn website. The trick is to uncheck the "prefer fast connections" box. Because these trains are actually slower than the 2 connection routes, they don't show up at first glance. The direct trains make additional stops at Brussels airport and De Haag and take 3 hours and 20 minutes vs 3 Hours and 2 minutes for the trains using connections.

I cannot get the site to show prices for the Berlin-Warsaw EC trains, but they do show the trains bookable inside 30 days. At www.bahn.com, they are bookable inside 90 days and show a Sparpreis fare of 29 EUR.

Seat 61 and various posters here recommend people to use the SNCB site to purchase rail tickets for journeys in the Netherlands in advance, as SNCB sell NS tickets and NS tickets are inconvenient and punitively surcharged to purchase by more official routes if you do not have a Dutch-issued bank payment card.