You completed the research to be able to build the trader before you got to the research for Adv Residence, so follow
@AtaguS instructions and you should be able to build it. Once built, you can post trades offering the marble you are making for the planks and steel that you need for the upgrades. Make sure to post the trades. Many large cities can see and take your trades with no fee and many do so on a semi-regular basis to help out new cities. When you're small, it's hard to find trades in small enough amounts that you can take with the goods you have available, so just post the trades and see if they get taken within a day or two.
You could, technically, build the planks and steel factories, but due to your boosted marble, it quickly becomes cheaper to trade marble even at a small loss than to take up the population/culture/space for those non-boosted factories. If you build them just to get past this point, you'll want to sell them off and get back the pop/culture space as soon as you can and use that pop/culture to upgrade your marble factories, making even more marble to trade for more steel and planks.
And so it begins...