How are you supposed to trade as a landlocked nation?

I'm trying to form Polish Lithuania, but there always comes a point in my playthrough where either Austria abandons me to my own market, OR the other polish states revolt and win, so I end up basically isolated with a very small state.

And from there... My economy just collapses because I have no exports anymore and my pops refuse to consume anything. My neighbours don't want to trade with me trough treaties, and trade centers require Merchant Marine (???) which is super expensive to buy because, again, landlocked.

Why can't I just create a trade route with Russia or Russia and export my steel, iron and coal there? Their prices are way higher than mine!