Based on context I assume we can place it between the Yalu river and Unsan

It may not be a city but maybe a geographic feature. Like mountains, hills, valleys, even the curve of a river, etc have names sometimes and that may have been one of them. At least I'm going off my understanding of naming as a westerner as those seem to be rules found all over the globe.

Considerations:

• It's possible the name was misspelled or lost in translation or the modern translation is different.
• It's also possible it's an old name and the area has since been given a new one. (Consider also it may have been a name used only by locals at the time or by the Chinese but not Koreans)
• It's also possible there is no name anymore because there's nothing there anymore so the name is lost.
• It could be a combination of these things.

The clue is I suppose it's something that can be reached from the Yalu river left at late evening time by morning via whatever conveyance they were using (walking, trucks, etc). I would see if you could find a copy of this in Chinese and see what the original language characters say there (deepl can help translating).

Its somewhere in this area and likely does not exist any more.

Raekosao is a very unusual name and likely based on revised romanization spelling.