My grandfather apparently was from a small village in eastern poland, called Rubel. This is now Belarus.
There must be a different way to spell it, as Rubel was the currency for Poland up until 1917.
Any clues or places to start looking?
There's this village called Rubel in Belarus. It would have been part of the Russian Empire until 1917 and then in easternmost Poland until 1939, then Belarus. Be careful, there could be other places of the same name though. You'd need more info, such as in which province it was, or any other data, to double check this village is really your Rubel.
Rubel is even visible on this map, marked "Rubl" very close to the eastern border, near the number "52".
With all research, but especially in this area, consider the time the record was written down -- the place name may vary over time. Historical maps and gazetteers (dictionaries of place names) may be more valuable to you than simply looking for a town of a similar name on modern-day maps.
You don't say how you know your grandfather came from this place. Was the information passed down via family history, or did it come from a record?
My first step in looking for clues would be to ask if he has siblings. Also think about whether other people in his community came from the same place (friends, associates, and neighbors). Examining their records may give you more clues.
Collect all the variant spellings so you can search for variants when looking at maps and gazetteers.
If you have only one record type, branch out and look for more (passenger lists, naturalization records, etc.). Looking at the records for multiple people and looking at multiple record types will give you more 'reach'. Start with records in the country the immigrant came to before looking for records in the country of origin, or in countries that immigrants may have passed through after emigrating.
Use the FamilySearch Wiki, the National Archives' website, and other sites to put the records in context. The more you understand the history and the purpose for which the record was created, the better you will be at spotting clues. One example from my: in the New England Naturalization Index, a naturalization index card that reports someone's country of allegiance as "Great Britain" may be describing someone from Ireland. In the same way, some people from modern-day Poland may be described as having come from Russia or Germany.
The FamilySearch Wiki has a series of articles on Tracing Immigrant Origins. If you get completely stuck, it can be useful to go back to the beginning and review everything, using the suggested search tactics as you review record groups in the country of arrival before tackling the country of origin.
JewishGen is very good at finding geographical names Behold Rubel', Belarus https://www.jewishgen.org/communities/community.php?usbgn=-1948821