Redmi K30 5G cannot connect to 5G signal

[jaychung]

Hi All,

I recently apply 5G from operator, it's used band n78.
But I can connect 5G via K30 5G.
First, I found MIUI 12 stable ROM didn't have prefer 5G option, I found same issue before the post, and use weekly ROM can fixed.
I install 20.8.6 weekly ROM, prefer 5G option had show up and I chooses it. but still cannot reach 5G.
I used *#*#4636#*#* to check information, I found the network type show "Unknown"!
Does anyone know how can I fix it?

Thanks!