[Xiaomi Redmi Note 5 Pro Questions & Answers] Aqq123:
Assuming you're still on anti-rollback 3 and not 4, and MIUI version below 9.5.17.0:

Right after you boot the phone, enable Airplane mode or switch off both Wi-Fi and Mobile data to cut all network access.

Go to Settings → System update and disable automatic downloading and installation of updates.

On your computer, download: http://bigota.d.miui.com/V9.5.17.0....roGlobal_V9.5.17.0.OEIMIFA_35b0cda8a7_8.1.zip

Copy the downloaded file to internal storage.

Set Settings → Sync → Auto-sync data: Off.

Re-enable Internet access.

Go to Settings → Mi Account and add the account you already have or create a new one. If creating an account, it's possible to only use e-mail (no phone number) if you click on a small link beneath the form.

Go to Settings → System update once more and point it to the file you put in the internal storage.

9.5.

17

.0 is the last version with anti-rollback 3 only. Double-check this is the one you are installing and let the system update itself. Do

not

install any later versions, including Developer ROM after version 8.7.5, unless you know what you are doing.

After the phone boots again, click on Settings → About phone → MIUI version several times until you see that "you're a developer."

Go to Settings → Additional settings → Authorization & Revocation and revoke authorization from Updater (you can also revoke all the other authorizations except Settings).

In Settings → Additional settings → Developer options, enable OEM Unlocking (a message needs to pop up, otherwise it didn't work) and USB debugging. (You can also enable Install via USB and USB debugging (Security settings) but for that you need to have a SIM card inserted.)

Click Settings → Additional settings → Developer options → Mi Unlock status → Add account and device. Wait for the message to confirm it worked.

At this point, you should have ADB and drivers installed on your computer. You don't need any special drivers beyond the regular Android ones.

Reboot to bootloader, either via adb reboot-bootloader or by holding the Vol- key while rebooting.

Download and unpack the MiFlash Unlock tool: from http://en.miui.com/unlock/download_en.html

Run the tool and log in with your Mi Account. It will fail to unlock the phone and tell you the remaining wait time, starting from 360 hours (or 15 days). As far as I can tell, there is currently no workaround.

This is the stage I am personally now at, so the following instructions are written from memory but afterwards it should all be easy.

Run the tool again once the countdown is over. This should finally unlock the phone. Note: all the data on the phone (including user data) will be deleted.

Download official TWRP from here: https://dl.twrp.me/whyred/

Install in bootloader (fastboot) mode with: fastboot flash recovery twrp-3.2.x-x-whyred.img

Boot into the downloaded recovery: fastboot boot twrp-3.2.x-x-whyred.img (or you can type fastboot reboot while holding the Vol+ key).

Download official LineageOS from https://download.lineageos.org/whyred

Once in TWRP, go to Advanced → ADB Sideload, swipe the slider right to enable it, and run: adb sideload lineage-15.1-2018xxxx-nightly-whyred-signed.zip

Reboot into system.

Other ROM options and the differences between them are very well described in the first post here: https://xdaforums.com/redmi-note-5-pro/how-to/vote-upcoming-custom-roms-redmi-note-5-t3750961