[Xiaomi Mi 4i] ashwaniv:
I was struggling very hard to make 4G/LTE work in my case. There is no standard method mentioned in this thread on how to install the ROM. Although I know the method is generally same in all cases but still there are some small variations which lead to different results for different people.

The process I followed was:

1. First installed MIUI 6.6.10.0 as instructed that the ROM will work on version 6 and below.

2. Then rooted the phone using Toolkit.

3. Installed TWRP recovery version 2.8.7.0.

4. Wiped data (factory reset)

5. Flashed CM13

6. Flashed Gapps (Arm64, 6.0, pico)

7. Wipe Cache/Dalvik Cache

8. Reboot system.

Now, the possible variation in above process are:

1. Starting ROM can be different (MIUI 5, 6, 7, CM12.1, RR etc)

2. Root method can be different (I am not aware about any other method than this)

3. TWRP recovery can be 2.8.6.0 or 2.8.7.0

4. Wipe data may include Factory Reset, or Format Data or advanced wipe (Dalvik Cache, System, Cache, Data, Internal Storage) etc etc.

5. CM13 version can be different

6. Gapps version cab be different (Aroma, Stock, Nano, Pico, 5.1, 6, Arm, ARM 64 etc etc).

7. Some may perform wipe cache/dalvik cache and some may not

8. The sequence of flashing may be different (Install CM13 and Gapps one after another without reboot or install CM13 first, reboot and then install Gapps and reboot or some other sequence).

[Edit] 9. Some people may be using single SIM and some are on Dual SIM.

In my case, I tried my best to make 3G/4G work but all in vain. Once I installed only CM13 without flashing Gapps and luckily 4G started working (tested on SIM 2) but then without Gapps, Android is half dead.

So, I tried different ways. I was not able to replicate the above success (3G/4G working) and again started experimentation.

Finally, I noticed that there is one setting in recovery in "Mount" where "System" box was not selected but "Cache" and "data" box was selected. So, I selected "System" box also and tried my luck.

Finally, this clicked for me and 3G/4G started working.

[Edit] Later on further research, I found out that this setting is not related to ROM flashing. It is related to USB connection. So, 3G/4G setting has nothing to do with this.

Now, following issues are still continuing:

1. Flashlight tile is playing hide and seek. Icon is not showing and when try to bring the same using "edit Tile" area, then it start showing and when exit the edit mode, it again disappears.

[Edit] Now it is showing. So, it is really a hide and seek and reason is not known. Anyways, not a major requirement and can be met with some other application.

2. 4G connectivity is not good. In my earlier ROM (MIUI 6 and 7, CM12.1 and RR), it was very good. Now the signal is fluctuating a lot and download speed it pathetic.

[Edit]: 4G/3G/2G changing is a problem only in dual SIM mode. It is working fine in single SIM mode. In Dual SIM mode, removing the SIM from tray, swapping the SIMs and then again putting it back helps to resolve the problem. In that also, first you have to select 2G and then again select back LTE for it to work. The first setting done immediately after sim insertion, will get freezed and later you will not be able to change it.

3. Light sensor is not working. After the call has been disconnected and phone is away from the ear, the screen display should automatically come back but it is coming back only after pressing power button.

4. Facebook is crashing. After deleting the file /data/data/com.facebook.katana/lib-xzs/libsslx.so , it started working. Facebook Messenger is still crashing.

[Edit] FB messenger also started working after deleting /data/data/com.facebook.orca/lib-xzs/libsslx.so

So, if you can mention what can be the possible mistakes people are making here, then we can also make the 3G/4G work like a charm.

Cheers!!

[Edit]: For flashlight, I installed a flashlight app from play store.

[Edit] Another bug identified is the screen display going to deep sleep. This happened to me only once as of now after 4 days of operation. The same happened in CM12.1 and RR also.

[Edit]: Also faced the issue of not able to attend the call 2 times. When sliding the call attend button towards the green button, call is not getting connected. Even it is not getting disconnected. This is not happening every time but occasionally.