ZTE Kis II Max main board diagnostics

Hi. Appears my main board (motherboard) is fried on my ZTE Kis II Max dual SIM phone. Symptoms were mild trouble with charging developing into severe issues and eventually impossible to charge. My best guess is my battery was gaining internal resistance (aka going bad from old age) and eventually the circuits involved in charging overheated and shorted?

Basically, eventually the phone would just get 'warm' when left on the charger for extended periods but appear to have no effect on the battery.

Finally tried some new batteries but the phone wouldn't turn on or charge (leading me to believe I had indeed 'fried' something?).

Eventually I charged my battery externally in another phone so it has a good charge and works fine in other phone, but my ZTE Kis II Max still won't even blip with the fully charged battery in it! So not only is it not charging, it's pretty much DEAD!

Except....

The phone gets warm while the fully charged battery is installed.. no lights, not vibrations.. totally unresponsive to any and all button combos and batteries in and out and hocus pocus what the cokus!

Something has juice flowing through it so I'm guessing something is fried, no?

How can I diagnose and or what parts could possibly need to be replaced on the main board? I got a guy that can do this man (in Yangon!). I just need parts! And I guess advice.