If the battery is under the seat, there should be a pull cable under the rear of the seat. You should be able to pull the cable and pick up the back of the seat and pull back on the seat to remove it.
The instruction manual should tell you how to remove the battery.
Have you tried removing one of the side panels ??. It may be in
there. On my bike the battery is in the same position as yours
and to remove it the seat has to come off but there are two
allen type screws on each side of the seat hidden underneath
near the back.