Donald Trump's uncle, John G. Trump, was a professor at MIT, but Ted Kaczynski never attended MIT and therefore was not his student.
Trumps uncle was at MIT, one of the great professors. Kaczynski was one of his students.

FactVerify
Source: factverify.com
Key Evidence
Ted Kaczynski attended Harvard University for his undergraduate studies and the University of Michigan for his graduate degrees; he never attended MIT where John G.
Trump taught.
Multiple fact-checking sources confirm there is no record or evidence of Kaczynski being John Trump's student.
timesnownews.comtimesnownews.comFact Check: Did Trump’s Uncle, Dr. John Trump, Really Teach Unabomber Ted...
the-independent.comthe-independent.comTrump, 79, forgets who is with him on Pittsburgh trip as rambling talk includes...
What the Evidence Shows
The claim consists of two main components: first, that Donald Trump's uncle was a professor at MIT, and second, that Ted Kaczynski, the Unabomber, was one of his students. The first part is true, John G. Trump was a professor and researcher at MIT with a notable career there.
However, the second part is false. Ted Kaczynski did not attend MIT; he completed his undergraduate degree at Harvard University and earned his graduate degrees from the University of Michigan. There is no evidence that Kaczynski ever studied or was taught by John G.
Trump at MIT or any other institution. Multiple reputable sources confirm that Kaczynski never attended MIT and thus could not have been John Trump's student.timesnownews.comtimesnownews.comFact Check: Did Trump’s Uncle, Dr. John Trump, Really Teach Unabomber Ted...
the-independent.comthe-independent.comTrump, 79, forgets who is with him on Pittsburgh trip as rambling talk includes...
en.wikipedia.orgen.wikipedia.orgJohn G. Trump This false association appears to stem from a misunderstanding or misstatement by Donald Trump himself during public remarks.
The claim that John Trump was the longest-serving professor at MIT is also inaccurate, but this does not affect the core falsehood about Kaczynski being his student. Given the clear educational history of Kaczynski and the lack of any credible evidence linking him as a student to John Trump at MIT, the claim is demonstrably false.