Reuben was the biological first born son of Jacob.
By rights he should have received the double-portion of land inheritance due the firstborn son. This did not happen because he defiled his father's bed. The account is mentioned when Jacob gives his end-times prophecies over each son.
3Reuben, you are my firstborn, my might, and the beginning of my strength, and the excellency of dignity, and the excellency of power;
4you went astray like water, you will not surpass others; because you went up to your father's bed; truly, you have defiled my bed by going up into it.
The idea of "defiling the father's bed" shows up in an important historical footnote. The Viking raiders, made Christian monasteries a common target of their raiding.
In the days before universities, monasteries were the place where God the Father reproduced in the world. The Vikings, the lost tribe of Reuben, in effect continued to defile the Father's bed for a very long time after the original Genesis account. This quality is a signature attribute of the tribe, and not just a fleeting footnote about Reuben himself.
The birthright land, the prize, was North America, and though the Vikings are widely regarded as being the first Europeans to visit the upper east coast of North America, it was not the Vikings who eventually settled, but the English, the lost tribe of Joseph, instead.
Three countries at the eastern edge of the Baltic Sea are also listed here as part of Reuben. Those countries are Estonia, Latvia and Lithuania.
The Estonian language is closely related to Finnish, part of the core countries of Reuben, so is easily included.
Latvia and Lithuania are more culturally distinct that Estonia, but both were overrun by the Swedish during the Norther Wars, at roughly the time of the seal for Reuben. For this reason they belong as part of Reuben in a tribal sense even with their distinct tribal history.
The modern desire to be separate from the Soviet spheres of influence, to be separate from Judah, is yet another reason why these 3 Baltic countries should be considered part of Reuben.