Hi,
The Brother QL-710w supports AirPrint and Wi-Fi Direct.
AirPrint can be used with an iPhone, providing that an AirPrint-enabled printer (About AirPrint - Apple Support) and the iPhone are connected to the same network (Wi-Fi router). AirPrint relies on Bonjour/mDNS to discover devices. The idea is to print without having to install apps or enter network data (such as IP addresses).
Wi-Fi Direct is a direct connection method without the use of a router. Since this is not AirPrint, direct printing to this printer from an iPhone, if at all possible, would require a special app. You may want to check details with the printer manufacturer. Could one of the apps (maybe something like iPrint&Label) at https://support.brother.com/g/b/downloadlist.aspx?c=us&lang=en&prod=lpql710weus&os=10004 be used?
An alternative solution could perhaps be to use a simple Wi-Fi travel router in order to create a local Wi-Fi network for the purpose of printing from the iPhone only. If so, AirPrint printing would work.