Guys do you remember the post by aeosilio? He suggested that we can replace/upgrade the battery with a custom build one. This is the post I'm referring to - https://rog.asus.com/forum/showthread.php?75154-G751-Battery-upgrade-rebuild.
I messaged him if he can build one for me as mine has the same 0% percent problem. I'll let you guys know if he replies or I csuccesfully change the battery.

My laptop had the same problem of charging upto 84% and shutting down if the charger was removed. This happened after I sent it to RMA and they replaced my battery with a faulty one. I contacted aeolisio about the problem and he told me I had two options
1. Get a new battery and replace it as the current battery might have some damaged cells and causing the battery to fail. To check this you can always go to bios and try to start easy flash and it'll say there is no battery or battery should be at minimum of 20%.
2. Rebuild the current battery pack by changing the cells.

I also read some posts about holding the power button down for ten seconds solving the blinking battery indicator and the not fully charging problem for some people and it was same for me. But that lasted like a month and after that it got worse, the laptop lasted only 30-35 mins with 84% charging and eventually completely shutting down without direct power. Also the battery used to get super hot - another indicator of failed battery I think.

Worst case scenario your motherboard is faulty and it's causing the problem. But from what I observed in the forum everyone got this problem about the same time so it's probably because of a bios update. So, I sent the laptop to aeolisio and he checked my laptop battery and told me that the battery was indeed damaged.
He advised me that getting a new OEM battery is the safest thing to do as the current battery may have faulty wiring that caused it to fail in the first place. So I ordered a new battery and he replaced it. He was quiet patient with my questions even did a thermal repaste as I requested. I got my laptop back today and I have to say my temps are stable - from max 93 while gaming to 74 max and the battery is properly working (fully charged to 100% and no overheating). Now I could have get this done by sending it RMA but I'm out of warranty and I have no idea what more problems are gonna arise after sending it to RMA.
So, if there are any guys with G751 out of warranty and have a battery problem like mine you could either do the testing yourself or send it to aeolisio. You have to pay for his services though, but he's quiet reasonable. He lives in California, luckily around 6 miles from my place so the whole process was smooth.