like mentioned here, you can engage in the trade in the map overview. You can laos find the succubus in the dungeon and pay her with expensive necklaces or expensive earrings and such i think, to improve the relation to a certain extent.
Map Overview that you build upgrades on for the country, conduct researches, build things ike lumberjacks and fisheries etc. Talk to one of your ministers in the throne room to access it.
if you scroll over to Dorgania, there should be a few areas that have a light blue square on top, indicating a possible trade option.
If you haven't done so already, there is another quest you can do that im pretty sure increases your relationship with Dorgania as well, talk to the Queen of Dorgania for it/