How much WP you got on there?
edit: just read you got 3*350wp
While that is plenty for your computer and routing, do you have any other electrical devices on board?
You have a tour of the vans inside?
Sure enough for the power consumption of the modems, but I don't think the 5G networks are solar powered in Australia. Using multiple networks at the same time will use more power than using only one.
Does a 5G/4G tower consume more energy when one extra device is connected to it (for redundancy), but not actually transferring data? The answer might be yes, but it'd be very marginal.