I’ve just replaced the power board on a Doogee Mix 2. The most stressful part is removing the back. Unless you are really good at it I would buy a new back cover and a new set of patch antennas from Aliexpress before you start. The back cover is about $6 and the patch antennas about $5.
Disassembly is the usual process of heating the back of the phone, ideally with a temperature controlled heat mat but you can use a heat gun if you are careful. With a very thin spudger you can pry the back cover away from the phone very gently, the glass will probably crack. Pry only in the lower half of the back cover. When you have enough room, insert some plastic shims (such as plastic playing cards) and use them to cut through the glue. On the Mix2 the battery is glued to the back cover, so you will have to be very careful to cut the glue between the battery and the back cover before lifting the back covery very much otherwise you will damage the cable connecting the battery to the motherboard.
Unless you are very careful you will damage the patch antennas at the top of the phone but these can easily be replaced.
Once the back is removed, unscrew the black plastic cover at the top of the phone (note one screw almost hidden by the camera bezel) and carefully lift it off, being sure to separate the fingerprint sensor so that the cable does not get damaged. You can now unplug the battery which you should do before attempting to replace anything else.
To replace the lower (charging) board, remove the screws at the bottom of the phone and carefully lift up the black plastic cover. Underneath you will find a small circuit board. Take a photo before proceeding further. Unplug the antenna jack on the left and the flat cable in the middle, unsolder the four separate wires and very carefully lift the board out before installing the replacement board, soldering the four wires back on in the same positions. and plugging in the antenna and flat cable.
Reassembly is the reverse of disassembly, except you will need some glue to attach the back cover (hopefully the original but probably the replacement if the original broke when you removed it). Aliexpress sell various glues with names like P6000 and P7000. Apply very sparingly to the edge of the cover only and avoid the antenna areas.