From the books

The established conventions in the books and the series lore are that bells were tolled before battle to man the walls (Battle of Blackwater) or to warn smallfolk of incoming danger ( Battle of the Bells ), not to surrender. The only established convention for surrender is "bending the knee", i.e. a defeated lord kneels to the commander on the opposite side as a sign of defeat, either in person or as a formality on paper. As per the established rules, the legitimate gesture of surrender is Cersei bending the knee to Daenerys and nothing less than that would have sufficed