Connecting pc to 4k tv

So when i plug my tv in it gets detected but it doesn't display; its 4k at what i thought to be 120hz but when i change the refresh rate from 60hz to 30hz in the Nvidia control panel, it displays no problem. Anyway to get a higher refresh rate to work? Maybe switch to display port? i have 3 high speed HDMIs running through 2 extenders.
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