I think you misunderstood the advice. Leave the cable attached to your derailleur. Then pull the cable manually, find a spot where it is exposed, like on the downtube. From the highest gear (smallest cog), you should be able to manually pull the cable enough to reach all of the gears while moving the pedals (a bike stand works best for this, or have a friend raise the rear wheel into the air for you). If it doesn't reach the biggest cog, then either something is bent or your low limit is stopping you. Loosen the low limit a lot, and see if you can reach it. Remember to readjust the low limit before you ride the bike, so you do not risk throwing the chain and derailleur into the wheel and either crashing, or doing serious damage to the bike, or both.

Take close up pics and post if you want someone to guess whether something is bent.