Using Wifi direct and 3G internet simultaneously

[Bluepeanut]

I have connected to an Epson XP-620 printer via wifi direct and been able to browse the Internet via 3G and print a web page. I then bought a Brother HL-L2365DW to do the same thing, where I have no broadband internet, but once I am connected by wifi direct to the printer I am no longer able to browse the Internet via 3G.

Brother support are telling me this is how wifi direct works - yet I am able to do exactly what I require on the Epson.

I'm struggling to understand how the same wifi direct technology can work differently on 2 different printers?