Many members think that MobileUncle Tool can backup and restore your IMEIs but you are mistaken friends, I have already tried, it does backup your IMEIs but doesn't restore the IMEIs.
If you are rooted then its better to backup your IMEIs as we all root to experiment with our device. Most of the advance member must be knowing the location of IMEIs in MTK devices, its in Data>NVRAM>md>NVRAM>NVD_IMEI, so just to be on safer side just copy entire NVD_IMEI folder on a safe place in SD card or in your computer. If you don't have backup of your IMEIs but do have backup of your ROM, then you can extract your NVRAM folder from backup as well. Just open the data.ext4.tar.a in 7zip and extract the NVRAM folder from there.
Though IMEIs can be written via Engineering Mode but sometimes it doesn't work without formatting the entire device. And formatting erases all the MAC IDs and then you have to use SN_Station to write your MAC IDs, and using SN_Station is really very painful.
So first try Engineering Mode, if it doesn't work then you are actually in trouble, now you have to write your IMEIs using one or the other IMEI writing application, but in this method you don't require any app apart from SP Flash tool.
Here is the method that too in just two lines.
1. Overwrite your backed up NVD_IMEI folder @ data>NVRAM>md>NVRAM folder, you might have to change the permissions in root browser, I prefer ES File Explorer.
2. Once you paste that folder successfully just flash your ROM normally using SP flash tool. Reboot your device and you would get your IMEIs back.