I'd recommend visiting your local uBreakiFix for walk-in repair services: https://www.ubreakifix.com/locations
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I am having this very same issue.
I had taken my unlocked by Samsung Note 20 Ultra 5G to uBreakiFix, but uBreakiFix couldn't run diagnostics on it due to the "mmWave module" being grayed out; therefore, uBreakiFix was not able to fix it.
I had also mailed-in my Note 20 Ultra 5G to that Samsung repair center in Irving, Texas, but they replaced the LCD screen instead of fixing the 5G "mmWave" networking issue. Seriously?
Samsung, I am having 5G "mmWave" networking issues, not screen issues. What you actually need to fix is the 5G "mmWave" networking issue. I have been dealing with this for well over a month now. Please, fix my Note 20 Ultra 5G properly!