Mathematically I have enough power but the power source don't seem like enough
Your 20 V, 5 A supply is capable of 100 Watts of power (V x I). You cannot get more power from the supply, whatever you do.

You can, however, convert the voltage. In the case of AC, you use a transformer. In the case of DC, there are many different conversion methods.

But, the maximum output power must always be 100 W! So, if you convert to 10 V, you can only get 10 A. If you convert to 1 V, then you can have 100 A. But the power won't increase. V x I can't exceed the original 100 W.

In the real world, each conversion will also have some inefficiency. So you'll actually end up with less power than you started with.