If it came with a USB charger, that's the only way on how to charge it. Unless if the other spektrum chargers charge through the balance cord.
It recognizes the battery as a 2s and the cell voltage due to the balance cord, but cannot charge as there is no main wires. That's how chargers like...
You only have the balance plug plugged in.
You also have to plug in the main plug (the one with 2 thicker positive and negative wires) into the charger too.
Looks like since its a Spektrum SMART battery, it can only charge with spektrum chargers.
Weird, that battery only has a balance port, but...