For some strange reason switches are identified by the number of terminals.

You are dealing with 3 way switches.

There are many different ways to correctly wire 3-way switches depending on where the power and where the light is connected.

But many of them are not wired correctly.

So just going by the colors does not help.

But on the switch you will silver terminals and brass or dark colored terminals.

Two of one color and 1 of the other color.

The color with only one terminal is the common connection. The color with 2 trminals are the traveler connections.

On the dimmer the same logic holds.

Thus the back is the common adn the reds are the two travelers.

From your description it sounds like you have a 3-way and 4-way switch. There should also be another 3-way switch on this circuit. The dimmer can only go in place of one of the 3-ways. Follow Bill's advice on how to wire it up.

thank you Billhart, it worked.

Thank you househelper, it worked.