According to one account, whilst Mondas was drifting through space, the Cyberman of the Faction emigrated to Planet 14, where they evolved separately from the CyberMondans who remained on the homeworld. Whilst on Planet 14 they pursued their goal of total Cyber-conversion, (PROSE: Iceberg) but were later driven away when the planet's own power was used against them, resulting in a catastrophic explosion. (PROSE: Scientific Adviser)
According to another account, Planet 14 was a designation given to Marinus. It was the fourteenth planet to have a damaged worldshaper and the Voord of Marinus quick-evolved themselves into the early Cybermen to survive global catastrophe before the Sixth Doctor and Jamie McCrimmon encountered them. Jamie destroyed the machine and caused a time field to change the planet into a landscape the Doctor recognised as Mondas. (COMIC: The World Shapers)
During the attempted invasion of Earth involving International Electromatics, the Cyber-Planner told Tobias Vaughn that both the Second Doctor and Jamie had been on Planet 14. (TV: The Invasion) He used some sort of trick to escape the planet. (PROSE: Original Sin) The Cybermen on Lonsis were aware of this incident. (AUDIO: Human Resources)
Facing the Cybermen on a Mondasian colony ship, the Twelfth Doctor recalled Planet 14 as one of the worlds where they rose, and that he defeated them there. According to this account, though both Planet 14 and Marinus had experienced the development of Cybermen, Planet 14 was a separate location from Marinus. (TV: The Doctor Falls)
Behind the scenes
As of November 2021, there is no officially recognised 14th planetoid in the solar system, much less a 14th planet.