Hi,
Which Mac operating system version are you using?
The Brother MFC-J6720DW is listed in About AirPrint - Apple Support, and should be compatible with macOS without additional drivers/software/apps, provided that the printer and the Mac computer are connected to the same network (Wi-Fi router).
AirPrint works with wireless or wired network connections. It may be advantageous to wire the printer by Ethernet to one of the LAN ports of a Wi-Fi router instead of using a wireless connection. The Mac and other computers/devices can still print wirelessly via the router.
If you are using the AirPrint technology, and there is a problem with device discovery, the following full power-cycle procedure (in this order, with these waiting times) may help:
Power off the Wi-Fi router, the printer, the Mac, and all other devices/computers. Wait two to three minutes. Power on the router. Wait two to three minutes. Power on the printer. Wait a minute or two. Power on the Mac. Finally, power on all other devices/computers.
These articles could perhaps be of interest to you:
Add a printer to your printer list so you can use it on Mac – Apple Support (UK)
Reset the printing system on your Mac to solve a problem – Apple Support (UK)