If the Setec is turned off or NOT connected to the 240v GPO, there will be no power to the internal transformer and rectifier within the unit itself to provide current to charge battery. To see if the Setec is working, disconnect your battery charger first and see if there are any wires coming from Setec connected to battery. If not, check that these loose wires have not shorted out. These wires should be connected to the POS and NEG of battery. Then connect the Setec with the 3 pin plug to a KNOWN live 240v outlet. If Setec working, two lights should come on on the front of unit. (If unit is a Setec ST20 Series III. From memory, if an earlier Series II model, only red light indicating 240 volt power available will light up). Red indicates 240v power available (why you can operate without battery while on 240v power) and a GREEN light which indicates Battery power available. As
@Drover said, when 240v power connected, the Setec will supply power to 12 volt accessories AND supply current from internal transformer/rectifier to charge battery. Max load including battery charging and lights/accessories is 20 Amps. I believe
@Drover has come up with why there are problems because of unknown or bad modifications that you were not aware of.