LiPo mode is generally what you want to use on our Lithium Ion pack, as long as the termination voltage is 4.20V.
The only difference between the two battery modes is the termination voltage. Older lithium-ion cell chemistries had a lower termination voltage, hence the need for a separate lithium-ion charge mode. Our battery is comprised of modern lithium-cobalt-oxide cells with a termination voltage of 4.20V just like lithium polymer packs, so lipo mode works fine. Some chargers with separate lipo and li-ion modes are designed for the older li-ion cell chemistries, terminate at a lower voltage, and do not fully charge our battery. Thus we recommend sticking to lipo charge mode.
If each of your cells are at about 3.56V, this does not add up to 18V. 3.56*4=14.24V. If your charger was reporting 18V, then something was wrong with it. This matches almost exactly to a 5S battery at 3.56V/cell, I suggest you check your charger is operating properly, as well as confirm the cell termination voltage is set to 4.20V.