Read post #435 and my prior post, this explains why.+1 for another Mi PAD 4+ user.
I am located in the USA and just installed 8.12.13 and do not have a SIM card inserted because I don't plan on using any cellular data with this tablet. I cannot see any 5ghz signal. If I insert my phone's sim card, the 5ghz Wifi radio starts working. I can connect to the wifi no problem. Then when I remove the SIM card and reboot the tablet without the SIM card the 5ghz wifi is gone again. Can you please look into this?