I noticed no one has actually written a post about this so I did.

Take Note: Flashing this ROM to your phone means unlocking your bootloader, and unlocking bootloader means losing your warranty. Try at your own risk.

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Redmi S2 finally received its own slice of Pie, but it's only available in China for now. But if you have been keeping up with the Xiaomi.eu community since the last few weeks, they already made the ROM available for users with unlocked bootloader.

A little info about the ROM
Device: Xiaomi Redmi S2 (ysl/HMS2)
Android Version: 9
Security Patch Level: 2019-05-01
MIUI Version: MIUI 10.3.2.0 PEFCNXM
Architecture: 32-bit / ARMV7

Aside from the major OS update, no new major features were introduced since this is still using MIUI skin. But little features include:
System-wide dark mode
Dual 4G (not sure if Dual VoLTE works since both of my networks here in PH don't support it yet)

But be warned, this ROM also bumps an update to your phone's modem hardware in order to enable Dual 4G feature so here are the consequences I've noticed so far:
Network signals turning from full 4G to no signal at all and v/v every few seconds
• Constant signal loss on SIM2
The only solution I can think of, so far, is always be in an area with way stronger network coverage, somewhere 4G signal is in full strength.

But as long as you're like me who is not that reliant to text and call services and always connected to WiFi, this can be your daily driver.

I still have a lot to tell that I might have forgotten so if you have questions, feel free to write a comment down below.

Link to Xiaomi.eu ROMs: https://xiaomi.eu/community/threads/miui-10-0-10-1-10-2-10-3-stable-release.47170/