LTE Modem in car

[ParticleX]

2 na options ko, dc-dc cigarette lighter at power inverter.

nakabili na kasi ako ng modem, mukhang feasible naman tong setup, hindi ko lang alam mga technical terms at computation

If you'll follow the DC-DC adapter route, get at least rated for 1000ma (or 1.0a) as according to your router's adapter. Unless may iba ka pang isaksak na AC appliances, sayang lang yung inverter for your setup. Check mo yung polarity, compare mo sa old AC adapter mo to the DC-DC one (selectable kasi eh) - usually center pin is positive.