Inefficient pag power inverter lalo na sobrang liit lang ng load
Easiest is to get a DC-DC cigarette lighter adapter from Concorde, Electronics Botique or similar. Dalhin mo na lang yung router mo to see if the tips fit. Yung may replaceable fuse ang kunin mo para di sayang - you can unscrew it near the contacts to check. Mga around 300 ito
Or get a portable router with Micro or Mini USB input - may nakita ako nito sa auntie ko Huawei din. Then get a branded cigarette lighter-to-USB adapter