GL-X1200: 4G bonding/balancing + External antenna

Does this router support load balancing over many connections? specifically, can i load balance over 2 x 4G connections?

My intention is to use this in a vehicle using 12V.

Is there a recommended limit to antenna cable length/type if i want to connect to antennas (both wirless and 4G) mounted on the outside of the vehicle?

You can load balance on the two modems.

No recommendation to change antenna. But you should consult professionals about antenna about this.