AirCard AC790S not charging,keeps reboot on wallplug

I purchased the 790s online last month, which it works perfectly fine for 3 weeks then suddently the router refuses to charge and boot while battery is inserted, even when using only the wallplug without battery the screen will flickers and it will randomly reboot.

I'm from Malaysia with no customer service center around, what should I do? As the seller recommends me to charge the battery with external charger and reset the device. I don't have an external charger and reset doesn't work.

Model version AC790S

Firmware NTG9X35C_01.03.28.05 ( somehow not updatable)