Wifi problem 5 GHz in 12 Pro Max?

I just got my iPhone 12 Pro Max 256 GB here in Indonesia on Friday 18 December 2020.

right from the start, I noticed that wifi speed is very slow, which means it is ‘choosing’ the 2.4 GHz band instead of the 5 GHz band.

I did the speed test from the same position where I used to sit with my previous phone, XS Max.

Maybe only 10% of the time, my iPhone 12 Pro Max can connect to the 5 GHz band and achieved full speed 200 mbps.

I am using mesh system, Netgear Orbi RBR50 and two satellites RBS50 so my house which is relatively small are all covered.

when I complained through Apple Chat on Sunday afternoon 19 Dec 2020, I was told to update iOS, but now the problem still persist (on iOS 14.3).

last night (Sunday 19 Dec 2020), I found a way to disable the 2.4 GHz band from the Orbi, and my iPhone 12 Pro Max suddenly cannot see the wifi (5 GHz only).

i have many other Apple Devices in my house, like my wife’s 11 pro max, my iPad mini 5, iPad air 2 — all can smartly choose 5 GHz because router/satellite is very close, and still can see the wifi (5 GHz only) when I disabled the 2.4 GHz for testing purposes.

I have tried all methods found online, from reset network settings, toggling off private address, toggling off wifi and networking location service in privacy settings, reset the iPhone (volume up, volume down, long press power until logo appears), rebooting the Orbi system, resetting the Orbi Router RBR 50 (pinning the back side of the router) et cetera et cetera.

I also found many users are having similar thing.

unfortunately I am in Indonesia , where the Apple Support is not that good, very sub-par. For a specialist, has to rely on other countries specialist, such as Spore. Returning unit is also going to be very difficult I suppose.

my case ID is 101273076112.

I have chatted/spoke to a couple of senior advisors, with no success until now.