This is what I've found on Etymonline.
early 14c., "real, actually existing, true;" mid-15c., "relating to things" (especially property),
from Old French reel,
from Late Latin realis "actual," [...]
from Latin res "matter, thing," of unknown origin. [...]
Meaning "genuine" is recorded from 1550s; sense of "unaffected, no-nonsense" is from 1847.
Real estate is first recorded 1660s and retains the oldest English sense of the word.
So, according to this source, "real" means "existing, true" as per the Latin origin, but then even the later meaning of "relating to things" seems to derive from the same root (through French).
Minor addition :
in Italian the term "reale" has the same double meaning it has in Spanish (royal and real), whereas this is not true for French (royal and réel), although the three languages all derive from Latin...
PS: Italian word "reale" has two meanings, because that word has two different origins, which phonetically produced the same word.
- from late Latin "realis,e" derived from Latin "res,rei" (thing). This means "real".
- from late Latin "regalis,e", derived from "rex,regis" (king). This means "royal". The loss of that "g" produces identical words. That happened through the influence to the old French "reial" (which had changed the sound [g] to [j], then [j] was lost in Italian), but also according to Italian grammar. Note that latin ending "-alis,e", which allows to obtain an adjective from a noun, is still active in italian as "-ale". Latin "regis,e" (accusative "regem") became the Italian "re" (it should have been "rege", which exists, but is only used in poetry and some ancient texts, the second syllab was lost early) , therefore "reale" is also automatically derived by "re" plus the ending "ale" without any reference to Latin, so that the loss of the "g" is consistent and makes the word regular. Notice also that "regale" exists as well in Italian (same meaning as "reale").
Sourcess: ethimologic dictionary, ethimologic dictionary, Treccanic encyclopedia