If the hard plastic pipe is really broken as you say, then don't connect other pipes, just replace it completely, otherwise it will leak easily, and a sewer pipe won't cost much.

I'll take my sewer pipe as an example to talk about how to replace it. As shown in the following figure

Step 1: Remove the connector of the left water tank.

Turn the bolt at the position shown in the above figure counterclockwise, and it can be easily unscrewed.

Step 2: Remove the connector of the right water tank.

Unscrew another bolt as in the previous step.

Step three, remove return pipe.

Screw the oil return pipe directly to the position shown in the above figure.

Step 4: Dismantle the whole sewer pipe. There is nothing to say about this step.

Step 5, install a new and identical sink sewer pipe.

When installing, be sure to wrap several rounds of waterproof tape (available in hardware stores, very cheap), so as to prevent water leakage, remember to remember.