First you need to connect your device to the PJ-673 WiFi network. This is Brother's documentation about connecting an iPhone/iPad to the 673 via WiFi:
https://download.brother.com/welcome/docp000760/pj673_ukeng_usr_01.pdf

Once your iPad is connected to the PJ673, tap on the Print button when viewing a web page (or other document) inside CalPrint.
Tap on Choose and select the PJ673 printer icon in the choose printer screen. If the PJ-673 does not appear, use the "+" button in the top right of the choose screen to add the PJ 673 manually. Enter the address described in the Brother installation document ie. 169.254.100.1

On the Print screen, tap on Setup to show the setup screen. If the "Brother Mobile" test is not showing, tap on "More" to add it. Tap on "Test" beside "Brother Mobile" to test the connection. Once the connection is confirmed, then the 'Brother Mobile' option should be selected in the bottom of the screen for Documents and Text.

Tap on the "Printer Options" button to choose your printer model and other settings like paper size and length.

After setup, you can simply tap on 'Print' with other documents after you have connected to the PJ 673 WiFi network.