I've never been offered anything but an annual premium with full payment due in advance, you may want to contact an insurer directly with your question.
I recommend that you review your agreement with the broker that you signed before joining the firm. If that agreement states that you are required to pay a full year before leaving you may be on the hook for it. There are many models, some brokerages will charge you a flat fee per transaction. Chat with them all to compare, that is all you can do... Good luck, sam