The word tradition in the Greek means literally things handed down. That’s all it means. Things handed down. And what he is saying then in that sense is this, stand firm and hold on to the things handed down, the things handed down through teaching by word and letter from us.
That’s divine revelation. In the Pauline letters and the revelation God gave him which he preached to them...the oral things and the written things.
You're sounding like a Catholic apologist.
So if the oral things were not written, where can one find them?
The answer is in the Church, the leaders of which were told to go into the whole world and teach all things that were commanded them by Christ. Mt 28:20