My neighbor gets an ADSL Router of ZyXEL P-660R form his friend.
However, he can not use it, for do not has a transformer-based adapter its that used is 9Volt at 1Amp, But he has only the 12V 2A.

He wanted to made the 12V to 9V converter circuit in the stable DC power supply form with some basic equipment

This circuit converts the power supply 12V, from a many sources such as battery 12V or the other, to the lower voltage output of 9V, and the maximum current at 2A.

In this circuit we use a plain components that easy to find and not expensive, the base of circuit are an op-amps(operational amplifier IC) uA741 and a power transistors 2N3055.

The NPN Darlington transistors (Q1-2N3704, Q2-2N3055) are used to boost up output current to 2Amp,and the 2N3055 can be hold with a big size heat sink as it very hot when used.

If we are short circuit or used a current over limit, this circuit is still usually. Because it has the Q3 stop automatically doing of Q1,Q2.

I think that this circuit is the way to solve your problem as well. Good luck.