I haven't seen almost anything of Vikings, but it's true there was a popular folkelore hero named Ragnar Lodbrok since there are multiple independent accounts of his story, mainly the Danish chronicler Saxo Grammaticus and the Icelandic Saga of Ragnar and his sons, both from the 13th century.

As for the historic Ragnar, he's not considered to necessarily have existed at all. Depending on which historian you ask what's true about him is somewhere between "nothing" and "very little". Most would probably agree on that it's not possible to determine what (if anything) in the stories are true. They are not at all reliable sources for that period; the main stories about Ragnar Lodbrok were written down 400 years after the events they purport to describe, which are in the early 9th century. They are full of fantastical elements and impossible claims (such as Hervarar Saga claiming Ragnars sons took over not just Sweden, Denmark and Norway but also England and Austrríki/Novgorod). The accounts are very inconsistent and don't agree even on some basic facts of Lodbrok's biography, and some of the common elements, like fighting giant snakes, are clearly fantastical.

The fantastical elements also include some standard fictional tropes like being killed in a snake pit. There's no evidence anybody was ever killed in a snake pit in Northern Europe and it doesn't seem very plausible (given the only venomous snake in the region is the common viper whose bites not generally lethal) but it's a common thing in Scandinavian folklore where for instance Gunnar in Atlakviða is put into one. Some literature about Ragnar (e.g. Krákumál) was composed as late as the 12th century as well, so there was still an active tradition of writing Ragnar stories by the time they were recorded.

No contemporary sources mention Lodbrok specifically. So, (and despite it being a not-uncommon name) just about every "Ragnar" of the 9th century has been identified with Lodbrok since the 13th century if not earlier. Such as Ragnar Sigurdsson of the poem Ragnarsdrapa, the Reginar mentioned in the Chronicle of Rimbert who supposedly took over administration of the convent of Turholt and the "Reginherus" who was supposedly the one who sacked Paris, among others. It's possible this or these historic Ragnars inspired the legend of Ragnar Lodbrok, or an existing legendary Ragnar was conflated with actual people.

The series "Vikings" is not based off facts of the Viking Age facts but rather the fiction of the era.